Are C-sections changing the maternal pelvis? Maybe, but this paper doesn’t show it.


A new paper in The Proceedings of the National Academy of Science is getting a lot of buzz.

According to the BBC, Caesarean births ‘affecting human evolution’:

[pullquote align=”right” cite=”” link=”” color=”” class=”” size=””]There is no cliff-edge in fitness. A mother who needs a C-section for a  7 pound baby could subsequently deliver an 8 pound baby vaginally.[/pullquote]

Researchers estimate cases where the baby cannot fit down the birth canal have increased from 30 in 1,000 in the 1960s to 36 in 1,000 births today.

Historically, these genes would not have been passed from mother to child as both would have died in labour.

Researchers in Austria say the trend is likely to continue, but not to the extent that non-surgical births will become obsolete.

In other words C-sections, by saving the lives of babies and mothers who would otherwise die and allowing for the persistence of genes for overly large fetal heads and overly small maternal pelves.

At first blush, the paper appears persuasive, but there’s a very serious problem here and I was not surprised to find that there were no obstetricians among the authors. That’s because the authors don’t seem to understand feto-pelvic disproportion:

The maternal pelvis is not a basketball hoop; the shape matters nearly as much as the size.

A very common cause of feto-pelvic disproportion is the position of the baby, not the size.

Both the baby’s size and the size of the mother’s pelvis are greatly affected by nutrition. Indeed, the apparent increase in feto-pelvic disproportion is far more likely to be due to improved nutrition than evolutionary pressure occurring over the minute time period of less than 100 years.

When you take these factors into account, the paper, Cliff-edge model of obstetric selection in humans by Mitterroecker et al. is not merely incorrect; it is foolish.

According to the authors:

The strikingly high incidence of obstructed labor due to the disproportion of fetal size and the mother’s pelvic dimensions has puzzled evolutionary scientists for decades. Here we propose that these high rates are a direct consequence of the distinct characteristics of human obstetric selection. Neonatal size relative to the birth-relevant maternal dimensions is highly variable and positively associated with reproductive success until it reaches a critical value, beyond which natural delivery becomes impossible. As a consequence, the symmetric phenotype distribution cannot match the highly asymmetric, cliff-edged fitness distribution well: The optimal phenotype distribution that maximizes population mean fitness entails a fraction of individuals falling beyond the “fitness edge” (i.e., those with fetopelvic disproportion). Using a simple mathematical model, we show that weak directional selection for a large neonate, a narrow pelvic canal, or both is sufficient to account for the considerable incidence of fetopelvic disproportion. Based on this model, we predict that the regular use of Caesarean sections throughout the last decades has led to an evolutionary increase of fetopelvic disproportion rates by 10 to 20%.

The authors reference “the obstetric dilemma,” the observation that there are two opposing evolutionary pressures on the relationship between the mother’s pelvis and the size of the baby’s head. In simple terms, a large head is an evolutionary advantage for a baby, but a large pelvis is an evolutionary disadvantage for the mother since it interferes with her ability to walk and run.

Not only are the needs of the mother and baby directly opposed at the time of birth, but the contribution of the father’s genes means that there is no coordination between the size of the mother’s pelvis and the size of the baby’s head, particularly if the father had a large head at birth.

The authors postulate an obstetric “fitness” function, D:

Successful labor requires the match of the neonatal head and shoulder dimensions with the dimensions of the maternal pelvic inlet, midplane, and outlet. Consider an idealized variable, D, that represents the difference between the size of the neonate and the size of the maternal pelvic canal. A negative value indicates a pelvic canal that can accommodate the newborn, whereas fetopelvic disproportion occurs if D > 0. In practice, this composite quantity cannot be inferred from the usual clinical measurements, but it is conceivable that D can be expressed as a function of a finite set of appropriate morphological measurements.

Using this idealized variable D, the authors created a mathematical model.

We present a mathematical model that explains the high rates of fetopelvic disproportion by the dis- crepancy between a wide symmetric phenotype distribution and an asymmetric, “cliff-edged” fitness function.

But there is no cliff-edge in fitness. There is are multiple factors that can be combined kaleidoscopically to lead to a variety of outcomes. To put it is real world terms: A mother who has feto-pelvic disproportion with an 7 pound baby and requires a C-section could subsequently deliver an 8 pound baby is a successful vaginal birth.

How can that be?

1. The maternal pelvis is not a hoop.

The pelvis is a bony passage with an inlet and an outlet having different dimensions and a multiple bony protuberances jutting out at various places and at multiple angles. The baby’s head does not pass through like a ball going through a hoop. The baby’s head must negotiate the bony tube that is the pelvis, twisting this way and that to make it through.


You can see what I mean in the illustration above (from Shoulder Dystocia There are bony protuberances that jut into the pelvis from either side (the ischial spines) and the bottom of the sacrum and the coccyx, located in the back of the pelvis, jut forward. How does the baby negotiate these obstacles? During labor, the dimension of the baby’s head occupies the largest dimension of the mother’s pelvis. But because of the multiple obstacles, the largest part of the mother’s pelvis is different from top to middle to bottom. Therefore, the baby is forced to twist and turn its head in order to fit.

This illustration (from the textbook Human Labor & Birth) shows what happens. We are looking up from below and the fetal skull is passing through the mother’s pelvis. The lines on top of the skull demarcate the different bones of the fetal skull.


Clearly there is a great deal of potential for a mismatch between the size of the pelvis and the size of the baby’s head. Over time, babies have evolved so that the bones of the skull are not fused and can slide over each other, reducing the diameter of the head. This is called “molding” and accounts for the typical conehead of the newborn. But there is a limit to the amount of molding that the head can undergo and ultimately, the baby may not fit through.

2. One of the most common causes of feto-pelvic disproproportion is the position of the baby, not the size of its head.

The illustration above shows the baby’s head entering the pelvis in the optimal position, but babies don’t always cooperate. If the head is in anything other than the ideal position the fit will be even tighter. That’s why babies in the OP position (facing frontwards) and babies with asynclitic heads (the head titled to one side) are much more difficult to deliver vaginally. Their heads no longer in the smallest possible diameter. It’s like trying to put on a turtleneck face first  instead of starting from the back of your head. It’s much more difficult.

And it’s far more difficult (and sometimes impossible) to deliver a baby vaginally if it presents brow first or face first. Moreover, 3-4% of babies are breech, meaning bottom or feet are coming first. The head is less likely to fit if the feet come first.

3. Genes are not the only determinants of the size of the baby’s head and the size of the maternal pelvis. Nutrition plays a critical role.

It is well known that the average size of babies is getting bigger, just as the average size of adults is getting bigger, as a result of improved nutrition. For most of the past hundred thousand years or so, humans lived a substistence existence and stunting of growth was common at all ages. Now, very few people starve in industrialized countries. Indeed, people are far more likely to be obese that at any time in human history. Obese babies have trouble surviving birth not merely because their heads might be bigger, but also because their shoulders are bigger and can get stuck during the process of birth (shoulder dystocia), a potentially deadly complication.

In contrast, better nutrition (and pregnancy delayed far beyond the teenage years) means that women who give birth are likely to have a larger pelvis and one that is not constrained by nutritional deficiencies like rickets.

Given these factors, the authors’ conclusions are fanciful.

The authors give a nod to other factors:

The success of labor is … influenced … by numerous other factors, including flexibility of the pelvic ligaments, orientation of the neonate, and efficiency of uterine contractions. However, as long as these factors are statistically independent of the discrepancy between neonatal and maternal dimensions, the selection gradient and evolutionary trajectory of D can be modeled independently of other factors.

That’s yet another faulty assumption. These factors are intimately intertwined. For example, the orientation of the baby is dependent of the shape of the mother’s pelvis. And the strength of the uterine contractions may be dependent on the size of the baby; a distended uterus may be less likely to contract effectively.

That’s why a mother who has a C-section for feto-pelvic disproportion with a 7 lb baby might subsequently deliver an 8 pound baby vaginally. That would be impossible if the cliff-edge theory of fitness were true.

And that doesn’t even take into account that C-sections have only become routinely survivable in the past 80 years or so, not even a blink in the eye of evolution and far too short a period of time for evolutionary pressures to have produced changes like those proposed.

Are C-sections changing the maternal pelvis? Maybe, but this paper doesn’t show it.

Indeed it doesn’t show anything at all.

76 Responses to “Are C-sections changing the maternal pelvis? Maybe, but this paper doesn’t show it.”

  1. omdg
    December 13, 2016 at 10:44 am #

    Nothing to worry about here, folks. If the apocalypse comes and c sections can’t happen anymore, moms will be so malnourished that babies will be smaller, so they will fit through the pelvis better. We’ll have plenty of other things to die from then instead.

  2. The Bofa on the Sofa
    December 8, 2016 at 9:38 am #

    I guess I don’t understand the point of the paper in the end. As far as I can see, it is merely an academic exercise. It cannot be any basis for policy, that’s for sure.

    Think about what it’s saying. OK, baby is to be born at time d. At that point in time, what difference does it make whether it is a c-section or a vaginal birth? The baby is the same size, regardless. The mother’s pelvis is the same size, regardless. Whether that baby is born either by c-section or vaginally at that time, there is no difference.

    Assuming it survives, of course. So the only way this has any effect is to do more of those deliveries vaginally, such that fewer babies will survive.

    Seriously, this is the implication. If this is an issue that needs to be considered, then the way to solve it is to make sure that those babies die.

    Now, there is an alternative conclusion that could be drawn, but it is another one of those that the NCB crowd won’t like. Instead of waiting to time d and letting the baby die, the other option is to induce earlier, before the baby gets too big. Of course, that won’t reduce the selective pressure, so it really doesn’t address the “issue.”

    Either way, I’m not accepting it. I am not willing to have my baby sacrificed to the altar of evolution, are you?

    • MaineJen
      December 9, 2016 at 4:10 pm #

      The types of comments I’m seeing are along the lines of “Well, we’re screwing ourselves for the future!” As if people are anticipating some kind of post-apocalyptic, post-technology scenario.

      Given the current political climate, I can’t say that’s unreasonable.

      But they do seem to be actively suggesting that yes, it’s better to let some babies die to preserve women’s ability to birth vaginally with minimal help. Which, yes, is insane. But that does indeed seem to be the implication.

      One might just as well say “We’re changing evolution by doing all of these appendectomies! The only way to allow humans to survive a post apocalyptic future is to let those people die, so only people born without appendices (sp?) will be left!” I mean, that’s insane, right?


      • Azuran
        December 9, 2016 at 6:59 pm #

        Totally insane. And the raise in C-section is never going to make vaginal birth impossible.
        Overall, we are increasing the odds that people who are ‘less’ likely to be able to give birth can have babies. So maybe the need for c-section will rise somewhat…because we didn’t let those people die.
        But we aren’t selecting AGAINST the ability to give birth vaginally.
        So just like doing appendectomy hasn’t resulted in a population where everyone needs appendectomies to survive, c-sections are not going to result in the inability to give birth vaginally.

  3. Gunter P. Wagner
    December 7, 2016 at 10:03 pm #

    That paper does not say that C-sections change the pelvis! It argues that high frequency of C-sections removes the selection against large neonate body size and thereby exacerbate the problem for which C-section was invented.

    • EmbraceYourInnerCrone
      December 7, 2016 at 10:35 pm #

      “removes the selection” nice phrasing for if we didn’t do C-sections then the bigger and big headed babies would tend to die off and voila! no more big or big headed babies growing up to breed more big babies. Which is bull anyway because C-sections happen for TONS of reasons not just macrosomia

    • Maya Markova
      December 8, 2016 at 2:56 am #

      I do not mind the problem being exacerbated, as long as we have a solution.

  4. Platos_Redhaired_Stepchild
    December 6, 2016 at 10:54 pm #

    As someone who comes from a family tree full of dead babies right up until hospital birth and vaccines became mainstream I’ll think I’ll take a hard pass on this eugenics inspired “natural” birth and parenting BS.

  5. CSN0116
    December 6, 2016 at 9:02 pm #

    This was an extremely complicated read. What is the background of the authors?

    • JD
      December 7, 2016 at 12:19 am #

      Look at her bio in the upper right hand corner of the page. She is an OB/GYN. The article was an analysis of a scientific paper and is therefore not going to be easy for everyone to understand. She could simplify it, but then she wouldn’t be able to go into as much depth.

      • Mattie
        December 7, 2016 at 6:23 am #

        I think CSN means the background of the authors of the paper…not the background of Dr Amy (although forgive me if I’m wrong on that count).

        • Empress of the Iguana People
          December 7, 2016 at 10:13 am #

          CSN does. She’s been a regular for much too long to not be aware of Dr. Amy’s bio.

          • Mattie
            December 7, 2016 at 1:07 pm #

            I thought so! But I didn’t want to have messed up with remembering a username wrong lol

      • CSN0116
        December 7, 2016 at 10:31 am #

        Oh, haha. Yeah, I meant the paper authors. I’m well familiar with Dr. T 😉 And I could have (and should) just google their names myself.

        I’m wondering how many journals they got rejected from before landing here, because this was just so oddly done and, imo, not well written.

    • Erin
      December 7, 2016 at 7:46 am #

      The person named first on the article is in his own words “a theoretical biologist with a strong background in physical anthropology, evolutionary biology, and statistics.”

      • CSN0116
        December 7, 2016 at 10:31 am #

        That explains the jargon. Ty!

  6. Tobus
    December 6, 2016 at 7:56 pm #

    This paper isn’t saying pelvises are getting smaller, it’s saying that babies are getting bigger: “One side of this selective force – namely the trend towards smaller babies – has vanished due to Caesarean sections”

    • JD
      December 7, 2016 at 12:21 am #

      But if the larger size is due to nutrition instead of genetics, then it wouldn’t be subject to selection.

      • Tobus
        December 7, 2016 at 2:46 am #

        We’re not talking about a new selective force here, we’re talking about the removal of an existing selective force – so whether it’s due to genetics or nutrition, it’s been subject to selection since the day dot and now no longer is.

        If the cause of head size is mainly genetic then we’d expect to see larger heads enter the population at a low frequency and fluctuate randomly over time since there’s no selective force acting on it. If the cause is nutrition then we’d expect to see the number of large heads grow and grow as more and more people have access to better nutrition. Presumably there’s a genetic limit to the maximum head size than can be obtained with better nutrition – in this scenario, how big this limit is would determine if caesarean births will eventually outnumber vaginal births.

      • MI Dawn
        December 8, 2016 at 8:50 am #

        Exactly. I can still recall the days – as I’m sure Dr Amy can – when weight gain for a woman was to be ONLY 20 lbs. If you gained more, you got scolded. Restricting nutrition to keep weight gain minimal can affact newborn size.

    • Sue
      December 11, 2016 at 7:32 pm #

      But the size of babies isn’t just genetic – it;s also influenced by nutrition and gestational age.

      These days, babies of a wide range of sizes, shapes and gestational ages survive. Isn’t medicine wonderful?

  7. guest
    December 6, 2016 at 6:35 pm #

    If there were any evolutionary change, it would be incredibly slow. Most women still birth vaginally, and not all c-sections are for large babies. (My own was done a month before my due date – we can never know if my babies would have fit through my pelvis since it was never tried, but had the induction worked they were certainly small enough to fit (so did they consider whether saving premature babies evolves larger babies, since those tiny babies *might have been* too big to fit if they hadn’t arrived early?), but they might have, meaning my particular c-section did not contribute to increased baby size in the future.

    • guest
      December 6, 2016 at 6:43 pm #

      What a mess my sentence ended up. Hopefully it’s not too unclear.

    • Jeklo
      December 6, 2016 at 6:45 pm #

      Ok cool, that makes sense.
      My sisters baby was enormous (10 pound 3!!!) and also 4 days overdue. They ended up using forceps and she came out with her hand up beside her face, she had a really weak shoulder for a while as a result. My sister doesn’t have what you would call a small pelvis either. She was determined not to have a caesarean but I will never understand why the doctor didn’t, I guess, suggest a lot harder that she have one.

      Edited to add: in the post it mentions that the fathers birth size matters too, and my nieces father was 10 pound 6. There was no mention before labour of a section and to me that just seems weird.

    • Daleth
      December 7, 2016 at 9:58 am #

      Seriously, c-sections have only been common for like 40 years. Human evolution does not work that fast.

      • guest
        December 7, 2016 at 11:15 am #

        Exactly. We might see some evolutionary shift in the case of a really severe disadvantage, such as a virus that some people are immune to, and those who are not die 99% of the time – a virus we somehow could not treat or slow the spread of. In a case like that, sure, we’d see clear natural selection going on. Those who are immune survive, those who aren’t, don’t. But those events are pretty rare. Most of evolution does not work that fast.

  8. Jeklo
    December 6, 2016 at 6:28 pm #

    Long time lurker but this is so interesting to me (as a lay person) I just really want to comment. I’ve always wondered about how we could evolve as a result of caesareans, how it could change babies and mothers. How far into life is a big head an advantage to a baby? Like in there any difference in intelligence or anything else between two five years old where one was born a lot bigger than the other? Could we eventually not be able to deliver vaginally at all coz babies have gotten so big? Could future generations be much smarter as a result? How many generations would it take before that would be noticeable? Is there a limit to how big babies heads could get even with generations of caesareans? Would there always be enough vaginal births to limit our whole species changing that radically?

    I just wonder everything but don’t have the scientific background to even begin trying to answer. Oh and if any of what I’ve said comes off as anti-caesarean rhetorical questions, it isn’t, when I go to have kids I’m open to any method of delivery that keeps them and me alive and well.

    • Valerie
      December 6, 2016 at 6:56 pm #

      My guess would be that the biggest selective pressure we have today is people choosing to limit their family size. We no longer have as many pregnancies as possible and then our best to get maybe half of them to adulthood. The days are over where only more intelligent people were able to make best use of their environments, avoid danger, exploit resources, etc, and survive. The fact that people choose to have fewer children and we have technology to ensure their survival probably dwarfs any greater “fitness” for bigger head size and smaller pelvises for populations with access to CS. There may be some drift over time, but without selective pressure, one way or the other, it’s not going to happen quickly.

      In other words, in 1000 years, the genes of the people who chose to have many biological children will be more prevalent than those of those who were unable or unwilling to have that many. It’s not going to be about pelvis or head size because (unless somebody has data to suggest otherwise) that doesn’t correlate to the number of surviving offspring anymore.

      • Jeklo
        December 6, 2016 at 7:06 pm #

        Ah yep I get it. So much stuff that would never occur to me, this is why I lurk here haha

      • JD
        December 7, 2016 at 12:28 am #

        The genes of people who have many children would only become more prevalent if survival was the same between those having a lot of children and those having few children. I think that since lower income people and people in developing countries tend to have more children and wealthier people and people in developed countries tend to have fewer children, it probably wouldn’t have much of an effect. It’s like how a dandelion puts few resources into many offspring, but a coconut tree puts a lot of resources into few offspring. Different strategies, but both species are able to survive and propagate themselves.

        • AnnaPDE
          December 7, 2016 at 9:12 am #

          Except that in first-world countries, both more and less wealthy people have a child survival rate of close to 100%, so the dandelion/coconut comparison doesn’t apply.
          If there is something genetic correlated with having many kids, that’ll be amplified.

        • Valerie
          December 7, 2016 at 10:04 am #

          They don’t have to have equal survival. Literally nothing matters besides how many descendants you have relative to other people- not how long they live, their quality of life, level of education, etc. It’s an empirical question- in countries with access to modern medical technology, on average how many grand- or great-grand children does a person have if they had 2 children? How about a person who had 6 children? You think they would be the same? For a person with 2 children to have the same fitness as one with 6, on average ~4/6 children would have to die before reproducing (as an effect of the larger family). This isn’t the society we live in anymore.

  9. December 6, 2016 at 6:01 pm #

    Polygenetic systems are a bitch to model – especially if you have no idea how many genes and alleles we are talking about.

    Modeling the genetic shift between maternal genotypes AA–>AB–>AC–>BB–>BC–>CC and fetal genotypes DD->DE->EE when DD can be born in all A’s, DE can be born in A’s and B’s and EE can only be born in AA and AB would be a bit of work assuming you have accurate data on the occurrence of all 18 paired combinations of maternal-fetal genotypes for the most recent births and want to see if you can determine how the population changed from an earlier estimated gene pool to get the higher CS rate today – but I’m pretty sure you could do it.

    It gets harder for every added gene or allele.

    It gets less accurate as the un-modeled environmental effect has more weight.

    It gets pointless if you don’t have the actual data on genotype distribution except as an academic exercise……

  10. AirPlant
    December 6, 2016 at 5:32 pm #

    So generational C-Section risk. Every single one of my female relatives on both sided have delivered their babies via C section. Both of my parents were born by section back in the 50’s so you know that shit was pretty serious. So I am going into babymaking assuming that a section is a very real possibility for me. I am managing my expectations accordingly, I want a planned section if there is even the smallest medical indication, an epidural ASAP if there is a TOL and I want my OB to understand that I am not even screwing around, anything looks like it is not going to plan, get the scalpel and get to work. My nightmare is that I am a guaranteed section, just like every other member of my family, and going through labor is just pain without purpose that may prevent me from being awake to see my imaginary future baby.
    My pregnant sister in law has a similar situation. Sections all over in the family, her own mother endured three days of unmedicated non productive labor with her first. My sister in law may as well be a Xerox copy of her mother so to my eyes her odds of an easy vaginal birth aren’t super great.
    Now I have already said where my thought process is, and I assumed that was the most rational path, but talking to my sister in law, she was telling me that she is planning for a vaginal birth and she doesn’t even want to consider the possibility of a section. So she wants to labor at home in water as long as possible. And she wants to forego the epidural because cascade of interventions. And really, her body, her choice, she is delivering in a hospital, I am sure they will take care of things appropriately.
    It just blows my mind. I have always thought that a section was pretty much my fate, that my genetics were crap and I should be prepared to go under the knife life my foremothers before me. My SIL’s thought process could not be further from that. It is giving me a lot to chew on.

    • Daleth
      December 6, 2016 at 5:57 pm #

      I have always thought that a section was pretty much my fate, that my
      genetics were crap and I should be prepared to go under the knife life
      my foremothers before me. My SIL’s thought process could not be further
      from that.

      This is just my take, but it sounds to me like you are using evidence-based thinking while your SIL’s approach is just wishful thinking. If she wants to labor at home in the water and fears the “cascade of interventions,” she has gotten sucked some distance into the woo. Someone with her family’s medical history on the one hand, and her woo-based expectations on the other, is at high risk for a traumatic birth experience simply because her chances of having the all-natural, uncomplicated childbirth she wants are unusually low.

    • maidmarian555
      December 6, 2016 at 6:48 pm #

      My mum had two natural births. My grandmother 5. My great-grandmother had 13. Two of my half-sisters (on my fathers side) had 5 a piece. My dad was one of 6. I come from an enormous family that has a history on both sides of numerous natural births and yet my own son’s head was too big to fit thru my pelvis which resulted in my having a caesarean. Whilst I would definitely recommend preparing for any eventuality, genes don’t always give you the result you were expecting.

      • Erin
        December 6, 2016 at 7:04 pm #

        I second this. Both sides of my family tree are full of tiny bird like women having 10 pound plus babies easily. I’m 5’8 and my six pound nine ouncer got stuck.

        I suspect the best attitude for pregnancy is to remind yourself why you are pregnant and to plan for every eventuality*. Oh and to avoid google if things go differently to the way you wanted them to.

        *If my plans don’t go to plan, I may need reminding of this.

    • AnnaPDE
      December 7, 2016 at 9:22 am #

      I can totally sympathise with this reasoning, and just went for a planned section without TOL. Good thing too, my maternal “square head” genes combined perfectly with the both-sides narrow-pelvis ones, and my husband’s family history of massive heads. So baby had his 97 percentile head wedged in my pelvis without labor even having started, and had to be pried out with forceps – so much for the “gentle” caesarean technique with “walking out the baby”. There was no way he could have fit. I’m not exactly disappointed that we didn’t try.

    • MI Dawn
      December 8, 2016 at 8:47 am #

      My mom had 2 really easy labors. Mine were “fast” (induced, a total of 12 hours (37 weeks) and 4 hours (36 weeks), respectively) but neither of mine were term. I’m curious to see how my daughters do. Fortunately, they are both level-headed and not into woo. So pregnancy, when it happens, will be handled as it needs to be, rather than according to some mystic perfection promoted on mothering dot com.

  11. niteseer
    December 6, 2016 at 5:29 pm #

    Does the paper give any mention of the fact that the world is a lot “smaller” these days, with populations interbreeding with each other that would have never come into much contact two hundred years ago? I can’t count the number of deliveries I attended, where an 80 lb Asian woman was giving birth to a 9 lb plus baby; sometimes vaginally, often by C-section. That is just one example, there are many instances of races that are very diverse in size. Sometimes I’d look at dad, and look at mom, and think……..oh, no……this is gonna be a challenge…….

    • BeatriceC
      December 6, 2016 at 6:10 pm #

      My mother’s OB probably had the same “oh, no” thought upon meeting my father. I described it in detail further down, but in a nutshell, he’s an NFL defensive linebacker type, and she’s a 90 pound scrap of nothing. I inherited my father’s size, my sisters inherited my mother’s. More complicated than a coin flip, but it could go either way.

  12. Heidi_storage
    December 6, 2016 at 4:19 pm #

    None of the parachuters will recognize it, of course, but this post shows Dr. Amy NOT promoting rah-rah-rah cesarean deliveries–she states, indeed, that a woman may be able to have a vaginal delivery of a larger child than the baby who got stuck and had to be delivered by cesarean. Basically: Birth can be complicated, and you can’t reduce it to simplistic factors like hip size or even baby size. This is something NCBers would never admit in their dogmatism.

  13. December 6, 2016 at 3:31 pm #

    Caesareans aren’t “causing” the issue – if women choose caesarean for no known medical reason, their children are no more likely to need a caesarean when they then go to give birth. So really this only applies to the group of women having caesareans for medical reasons – in which case the alternative is death or foregoing biological motherhood…
    There’s something awful that it’s “the shape of a woman’s pelvis” that matters – passing a baby is such a small part of being a woman, or a mother that focussing on this and demonizing the work around seems so wrong.

  14. CanDoc
    December 6, 2016 at 3:10 pm #

    Okay, so let’s say the paper is correct and that our pelvic shape will change as natural selection stops killing off the offspring of women who have a small pelvis. So what? What are we going to do about it? Start telling women that if they need a cesarean section for obstructed labour that they must have a tubal ligation at the same time to avoid further promulgating their “defective pelvis” genes? Of course not. Let’s just be glad that we have the means to see infants safely into the world.

  15. mabelcruet
    December 6, 2016 at 2:57 pm #

    Fetal growth is a hugely complicated area-there are numerous genes involved and we know very little about how they interact, and how much environmental factors interplay with the various genes. Essentially, though, the dad’s biological imperative is to grow the biggest baby ever, the bigger the better, because dad’s genes are like ‘breed a big baby, baby gets best chance to survive, my genes will pass on and live forever more’, whilst mum’s genes are going ‘shit, I have to deliver this-eek!’, so mum’s genes inhibits growth rather than promotes it. It’s the parental conflict hypothesis-it always seemed to make sense to me.

    • Roadstergal
      December 6, 2016 at 3:30 pm #

      It’s more complicated than that. Genetic combinations that give a girl a wider/more flexible pelvis would be selected for on the basis of survival of childbirth, but that’s a complex combination that probably involves genes that are being selected for in other ways over here and over there (and of course, the survival advantage of having a narrower/sturdier pelvis for running would have to balance out). Genes for bigger brains/heads would be selected for to a point, because there’s a certain death/survival balance that would come out as optimal, and again – complex genetics that probably aren’t devoted simply to brain and head size. Bigger bodies might be selected for but might not, because bigger isn’t always better.

      It’s not as simple as biological imperatives driving genes, because men and women pull from basically a common genome. Your grandma’s genes are in your dad, after all, and granddad’s are in mom.

      • mabelcruet
        December 6, 2016 at 4:21 pm #

        I know it’s really complicated, that’s why I said ‘essentially’!

    • BeatriceC
      December 6, 2016 at 4:52 pm #

      My own birth is a perfect example of these competing selections. My father’s family is populated with people who really belong on an NFL defensive line. Even the women, me included, are large. My mother’s family is tiny. The men are generally tall and thin (six foot or so but 30/32 waistbands), and the women are generally short to medium height and extremely thin. My mother was 5’4″ and 89 pounds prior to getting pregnant with me. My maternal aunts and my sisters are skinny like this too (my niece too…she finally broke 100 pounds at age 21 and 5’6″ and eats like a damned elephant). Then there’s me. I weighed 10 pounds, 9 ounces at birth (term…due date was 12/25, I was born at 11:58 on 12/24). There was simply no way I was coming out the traditional way, but boy did they try. After a little over a day of labor, they called the section. Prior to safe c-secitons, neither my mother nor I would have survived. I was just way too big to fit through her tiny pelvis. My sisters’ genetics were kinder to my mother.

  16. Valerie
    December 6, 2016 at 2:50 pm #

    Unless I’m totally missing something here, there is another glaring problem with extending their model to “predict” the current discrepancy rate. It assumes that there is still an increased fitness of a larger baby or a smaller pelvis, and they don’t show that this is the case. We currently live in a society where we expect nearly every healthy term (or near term) fetus to survive to adulthood, so a lot of selective pressures have been lost- even those that favor a smaller pelvis or a larger baby. Eg, tiny babies are surviving and thriving because doctors and researchers have made huge advances in how to care for premature infants. The various problems of a large pelvis they highlight in the paper no longer interfere with a person’s survival or reproductive capability. The authors would have to show that modern women with narrower pelvises and babies born larger are having more children than their larger-pelvised, smaller-born counterparts. Seems pretty far-fetched.

    Also, it adds smoke to this movement that demonizes CS, even though it has nothing to do with it. In this model, “unnecesareans” have no direct effect. The only way for CS to affect the population is for mothers and babies who would definitely have died otherwise to survive- which I think everybody agrees is a good thing.

  17. Ash101
    December 6, 2016 at 1:20 pm #

    OT – 2nd night and breast feeding v formula feeding

    Can supplementing with formula the first few days prevent the dreaded Second Night fussiness?

    LO was soooo upset on the 2nd night. We were bfing, and she ended up losing 9% of her body weight. Doctors told us it was fine, but now I wonder if the 2nd night was so bad just because she was hungry and my milk hadn’t come in.

    Whenever I have baby #2, I’m planning to supplement with formula those first few days to prevent a repeat of that level of weight loss and hunger/suffering. I was thinking it would also prevent the dreaded 2nd night. Is there any evidence of this?

    • CSN0116
      December 6, 2016 at 1:29 pm #

      Yes, it does. Anecdotal, of course. God knows there exists to study to prove it.

      EFF newborns do not experience this 2nd night fussiness that I have ever heard of (unless mom is withholding the full amount of milk that baby wants to eat because she has been told “his tummy can only it THIS much milk”).

    • RNMeg
      December 6, 2016 at 1:59 pm #

      I supplemented my second baby from birth, as you’re planning on doing. He was a much happier, quieter baby than my daughter had been. She lost more than 10% of her body weight and we ended up staying an extra day in the hospital (going through a punishing pumping schedule because, of course, god forbid we give her some formula). My baby boy only lost, I think, 5-6 ounces before he started gaining again and we were discharged on time with much less stress. Oh, and, despite the evil supplementing we did, I ended up breastfeeding him for 8 weeks until I went back to work.

    • maidmarian555
      December 6, 2016 at 5:25 pm #

      Once again, anecdotal, but I combo-fed from day one and it worked very well for us. Particularly whilst I was recovering from my c-section when Dad could get up and do the 3am feed so I could get a bit of rest. My son was a large, VERY hungry baby and I don’t think we’d have got through those first days as well without supplementing.

    • TheArtistFormerlyKnownAsYoya
      December 6, 2016 at 6:42 pm #

      We did not have 2nd night fussiness, and it was on day 2 we supplemented with a bit of formula. I didnt even know about “2nd night fussiness”, we just thought he must be hungry!

    • AnnaPDE
      December 7, 2016 at 9:26 am #

      I wish I had supplemented that night instead of listening to the lactivists. The crying was obviously from hunger and once we got the bottle of formula in the morning it stopped in a split second.

  18. Mrs.Katt the Cat
    December 6, 2016 at 12:44 pm #


    Anyone care to join me in a comments section??

      • LibrarianSarah
        December 6, 2016 at 5:24 pm #

        It was not. *angry face*

        • Ash101
          December 7, 2016 at 8:33 am #

          Very well then, I shall take them on via facebook. Assholes.

        • MI Dawn
          December 8, 2016 at 8:42 am #

          Try again with no links.

      • Mrs.Katt the Cat
        December 6, 2016 at 6:17 pm #

        Thanks, that was much more detailed than i had time for.

      • MI Dawn
        December 8, 2016 at 8:42 am #

        Huh. Interesting. My post went through with no problems. But I didn’t put in any links.

        • Ash101
          December 9, 2016 at 8:14 am #

          I posted the comment on the NBC4i and WWLP articles – neither were accepted. My cynical side screams “CENSOR MUCH!!” but maybe you’re right and it is the links. Then again…how are you supposed to back up what you say without links? Are they seriously forcing people to debate their points without the use of accessible references? Do they want me to include a link-free bibliography? This is the internet, not my dissertation 🙂

          I’m not too worried though, since I ALSO put the comments on NBC4i and WishTV’s facebooks of the article and those are still up. But perhaps I’ll try just one more time sans links and see how it goes…

        • Ash101
          December 9, 2016 at 8:32 am #


  19. maidmarian555
    December 6, 2016 at 12:43 pm #

    A couple of thoughts as a layperson- this paper has been widely reported here in the UK. They were talking about it on TV this morning. Before I had my son, I had no idea that it was even possible to have a baby with a head too large to fit through the pelvis. I don’t know anybody other than me that it’s happened to. If this had been released and reported on before I had my baby, it would have certainly been less of a shock when it happened to me. In addition, one of the authors of the paper was speaking this morning about how the number of babies with too-large heads seems to be on the rise whereas previously it’s been treated as a static percentage. He said they hoped that by highlighting this information that it would potentially be something that was taken into account when organisations and medical professionals talked about ‘optimal c-section rates’. I didn’t think that was necessarily a bad thing either.

    • Erin
      December 6, 2016 at 1:01 pm #

      Pushing and forceps couldn’t get my son’s head through my pelvis. He started back to back but they thought he was in a semi optimal position by the time I pushed. Had zero effect whatsoever.

      Nor was I told that there were different types of pelvis, I only discovered that mine was “flat” when the Midwife told the Doctor when they were trying to figure out why pushing wasn’t having any effect.

      • maidmarian555
        December 6, 2016 at 1:32 pm #

        I think I’ve said before how it made me feel really crappy after I got home following my son’s birth and Googled CPD, only to find a bunch of NCB pages where they said it was something that doctors pretty much invented in order to trick women into have unnecessary c-sections. I was actually pleased when I saw this paper reported this morning because I hoped that it would at the very least least raise awareness that this is a very real thing that does happen to some women. I don’t know that I would have let them drag my induction on for three days without question had I known it could happen.

        • Erin
          December 6, 2016 at 1:58 pm #

          I can understand and sympathise with that. I spent hours googling, trying to figure out what had always been described as “childbearing hips” had failed me so badly.

          My son was born 81 hours after my waters broke because I trusted the midwives who told it despite evidence to the contrary that it work with time. I thought people only had sections because their babies were breech or sideways or in distress. I didn’t realise that I could have a baby who tolerated labour just fine but couldn’t be pushed or pulled through my pelvis.

          My biggest regret is that I didn’t listen to my instincts which was to demand a Doctor much earlier than I actually saw one. I think I would have found the aftermath of my son’s arrival so much easier to deal with, had the issues which were picked up on in my notes been relayed to me at the time rather than me discovering them 8 months later when I accidentally ended up with a debrief.

          Interestingly enough when I raised that, they told me it would have disheartened me and I might have given up. I could have punched her, in fact thinking about it, I still could. Apart from the fact that I’m the sort of person who takes being told I can’t do something personally, that’s like printing out the consent form, cutting it into 30 jigsaw puzzle pieces, chunking 11 of them in the bin and then still asking you to give “informed” consent.

        • The Bofa on the Sofa
          December 6, 2016 at 1:58 pm #

          I thought the same thing. So this paper is saying that, yes, women CAN grew babies that they can’t give birth to?

          NCBers can’t be happy about that.

          • Roadstergal
            December 6, 2016 at 2:07 pm #

            It also is indirectly saying that if you don’t have an indicated C-section, you or the baby or both could die. Because that’s the way selection works.

  20. Amy M
    December 6, 2016 at 12:18 pm #

    Is it true that the rates of breech and asynclicity (is that a word?) are higher for multiples? And if it is, would that be because the babies can obstruct each other from lining up more ideally? I realize that this paper doesn’t mention multiples at all, but that could be another environmental factor that shows how important positioning is.

    • Daleth
      December 6, 2016 at 6:02 pm #

      Part of it is just because there are two (or more) of them, which doubles (or more) your chances that one will be in a suboptimal position. But yes, I do think that they get in each other’s way and move around to find a comfortable way of coexisting. My twins were vertex and breech like yin/yang in almost every ultrasound we got.

      • Amy M
        December 7, 2016 at 10:45 am #

        Mine were both basically vertex (by the time of birth), but the first one (Twin A), had his head turned to one side. I’ll never know for sure, but I always imagined it was because he was pretty much jammed in there by his brother and couldn’t move. He had to be pulled by vacuum, but then his brother simply slipped into ideal position after the space was vacated, and was born quickly and easily. Twin A was the bigger one, also, at a whopping 5lbs 2oz.

    • guest
      December 6, 2016 at 6:42 pm #

      But multiples are also prone to smaller birth weights (and that’s even discounting prematurity), which might partially offset that effect.

  21. fiftyfifty1
    December 6, 2016 at 11:55 am #

    “could subsequently deliver an 8 pound baby is a successful vaginal birth.”
    maybe should be
    could subsequently deliver an 8 pound baby IN a successful vaginal birth.

    “trying to put on a turtleneck face first of over your ear ”
    delete the “of”

    • Amy Tuteur, MD
      December 6, 2016 at 12:27 pm #

      Thanks! Fixed it.

Leave a Reply

You must be logged in to post a comment.