Has improved nutrition made childbirth more dangerous?

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There’s one pregnancy intervention that everyone — midwives, doulas, childbirth educators, obstetricians — can agree on: promoting optimal nutrition.

We encourage women to get all the calories necessary to grow a baby as well as the full daily requirement of vitamins and minerals. We assume that will improve pregnancy outcomes by improving the health of mothers and babies. We haven’t stopped to consider that there’s more to improved nutrition than what is obvious on the surface. What if nutrition it is making childbirth more dangerous because babies are bigger?

[pullquote align=”right” cite=”” link=”” color=”” class=”” size=””]For most of human existence, babies were probably much smaller than they are today.[/pullquote]

Midwives and natural childbirth advocates are known to bewail the high modern C-section rate by pointing out that childbirth can’t possibly required a 32% surgical delivery rate or our species could not have survived. Leaving aside for the moment their faulty understanding of evolution, they have failed to consider a more basic reality. Childbirth today is very different from childbirth in nature because the human diet is very different from our diet in nature. For most of human existence, babies were probably much smaller than they are today.

That has important implications for both mothers and babies. On the plus side, mothers are healthier with higher blood counts and therefore better able to withstand the rigors of labor and subsequent blood loss. Furthermore, nutritional rickets, which often led to contracted maternal pelvis making it impossible to deliver a term baby, is almost non-existent in industrialized countries.

On the minus side, the risk that a baby will grow too large to fit through the maternal pelvis leading to obstructed labor and the death of mother and baby has almost certainly increased. In modern societies we bypass that deadly result with C-sections.

A 2012 study in the Journal of Pediatrics shows that newborn size has been increasing.* Eighty year trends in infant weight and length growth: the Fels Longitudinal Study found:

Infants born after 1970 were ~450g heavier and ~1.4cm longer at birth, but demonstrated slower growth to one year, than infants born before 1970. Growth trajectories converged after one year of age.

Recent birth cohorts may be characterized not only by greater birth size, but also by subsequent catch-down growth. Trends over time in human growth do not increase monotonically, and growth velocity in the first year may have declined compared with preceding generations.

Newborn infants born in the years after 1970 are an average of 1 pound heavier than those born in the 40 years prior to 1970. Why?

[F]actors that have been responsible include changes in maternal biology and health (including a reduction in smoking prevalence and improved nutrition unrelated to maternal BMI or heights), an improvement in socioeconomic status and living conditions, and reductions in poverty and better provision of, and access, to health care and education.

Once these bigger babies are born, however, their growth rate is slower than babies of previous generations resulting in a convergence of size at the age of 1 year. This observation further strengthens the hypothesis that it is something about pregnancy, not babies, that has changed.

Another possible downside of increased neonatal size is that a bigger baby may be more likely to outstrip a placenta’s oxygenating capacity making that baby more vulnerable to distress in labor or stillbirth. The US stillbirth rate has not risen; indeed it has gone down, but that has happened in parallel with a dramatic increase in C-section rates and induction rates, allowing for rescue of babies that would otherwise die.

The hypothesis that improved nutrition has made childbirth more dangerous is speculative, of course, but it could explain a lot of observations that confound midwives and other natural childbirth advocates. It explains why intervention rates have risen: pregnancy itself has become more dangerous to both mothers and babies. It explains the results of studies like the newly published ARRIVE trial that showed that inductions not only improve outcomes but lead to lower C-section rates (a 39 week baby is both easier to deliver and less likely to experience fetal distress than a 40, 41 or 42 week baby). It may also explain why we are hearing more about postpartum pain, incontinence and discomfort during sex and long term incontinence and pelvic prolapse.

Who could disagree with the idea of improving nutrition for pregnant women? No one, but that doesn’t mean it isn’t an intervention. And it’s an intervention that may have made childbirth more difficult and dangerous — a consequence we haven’t considered because other inverventions have allowed us to avoid the potentially deadly results.


*That trend seems to have reversed in the past two decades with babies becoming slightly smaller, but still bigger than one hundred years ago.