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.