New sibling study shows C-section does NOT increase the risk of childhood obesity

fat child check out his body fat with measuring tape

In the ongoing effort to demonize C-sections, the association between C-sections and childhood obesity has received a lot of press. There have been numerous efforts to show that C-sections cause future obesity and a complex mechanism involving the gut microbiome has been proposed.

But there’s always been a serious problem with such research; the failure to adequately correct for confounding variables. We know that maternal obesity is a risk factor for C-section and we also know that maternal obesity is a risk factor for childhood obesity. Does the purported association between C-section and obesity mean that C-sections cause of obesity or that maternal obesity causes child obesity?

The risks of C-section have been overstated because of failure to fully correct for confounding variables.

A new study in JAMA Pediatrics attempts to address that issue by looking at the impact of C-sections within families. Association of Cesarean Delivery With Body Mass Index z Score at Age 5 Years is an analysis conducted by researchers from Harvard and the NIH.

The authors explain the current state of research:

Two meta-analyses, summarizing data from 24 studies, have reported an increased risk of obesity for individuals with cesarean birth (pooled odds ratio [OR], 1.22 [95% CI, 1.05-1.42] and 1.33 [1.19-1.48]).1,2 Limitations of earlier studies include small sample size in several studies and lack of adjustment for maternal body mass index (BMI [calculated as weight in kilograms divided by height in meters squared]) and sociocultural factors. Even after adjusting for these measured maternal characteristics, residual confounding is likely.

And propose a solution:

Within-family analysis is one way of controlling for such confounding. Because siblings grow up in similar social, economic, and cultural environments and share the same genetic predisposition to obesity, sibling studies minimize the variation in several of the noncausal factors that could explain why cesarean delivery could appear to be associated with a higher risk of obesity.

What did they find when they conducted within-family analysis?

Mean BMI z score was 0.45 among siblings who both had vaginal delivery, 0.51 among siblings with 1 cesarean and 1 vaginal delivery, and 0.63 among siblings who both had cesarean delivery.

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What does that mean?

We found that, within families, cesarean delivery was not associated with higher BMI z score at 5 years of age. This null finding suggests that confounding by unmeasured variables, such as maternal BMI and sociocultural factors, accounts for observed associations between cesarean delivery and BMI z score in some earlier studies…

C-section does not increase the risk childhood obesity, but rather the maternal characteristics that lead to the need for C-section are responsible for the observed increased risk of childhood obesity.

That’s consistent with a variety of recent studies.

Maturation of the infant microbiome community structure and function across multiple body sites and in relation to mode of delivery found:

…[T]here was no discernable effect of the cesarean mode of delivery on the early microbiota beyond the immediate neonatal period (and never inclusive of that in the meconium or stool) …

A Critical Review of the Bacterial Baptism Hypothesis and the Impact of Cesarean Delivery on the Infant Microbiome showed:

Although most studies report no differences in the microbiome of VD and CSD neonates in the first days of life, evidence is compelling that differences begin to develop shortly thereafter and persist for weeks or months.

That suggests that it is not the mode of delivery that contributes to the difference.

Mother-to-child transmission of obesogenic microbes continues to disrupt microbiome patterns into early childhood. Galley et al. found that the gut microbiomes of toddlers born to obese mothers of high socioeconomic status (SES) clustered away from those of toddlers born from lean high SES mothers…

It’s not the C-section but rather the mother’s microbiome itself.

Sharp eyed readers will recall that it was a within-family study that put to rest the notion that breastfeeding has massive health benefits.

Is Breast Truly Best? Estimating the Effects of Breastfeeding on Long-term Child Health and Wellbeing in the United States Using Sibling Comparisons by Colen and Ramey, showed:

When we more fully account for unobserved heterogeneity between children who are breastfed and those who are not, we are forced to reconsider the notion that breastfeeding unequivocally results in improved childhood health and wellbeing. In fact, our findings provide preliminary evidence to the contrary. When comparing results from between- to within-family estimates, coefficients for 10 of the 11 outcomes are substantially attenuated toward zero and none reach statistical significance (p < 0.05). Moreover, the signs of some of the regression coefficients actually change direction suggesting that, for some outcomes, breastfed children may actually be worse off than children who were not breastfed.

In other words, the benefits of breastfeeding had been dramatically overstated because of failure to fully correct for confounding variables.

Similarly this new within-family analysis suggests that the benefits of vaginal birth have also been dramatically overstated because of failure to fully correct for confounding variables. C-section does not increase the risk of childhood obesity. The risk is increased because of the factors that led to the C-section, not the C-section itself.

  • AnotherOor

    It seems so obvious – obese parents are likely to have obese children. News at 11: water is wet.

  • lawyer jane

    Excellent. Can we move on and start to work on actual ways to help prevent obesity? Like making healthy food easier to prepare and more widely available; getting junk food out of schools; getting kids more opportunities to exercise?

  • fiftyfifty1

    This is terrific–finally a well designed study that can give us a reliable answer. This *should* put the question to rest, except that it won’t if the breastfeeding example is any indication. Despite excellent breastfeeding studies like the sib study and PROBIT, we are still subjected to myriad poor quality, heavily confounded breastfeeding studies that inexplicably find a journal to publish them and inexplicably find a crappy journalist to report them in the lay press. So frustrating!

    • Sarah

      Mmm, my first thought on reading your opening sentence was ‘that’s very optimistic of you’! But we need to keep making sure this sort of thing is out there and is part of the discussion.

  • BeatriceC

    It’s almost like genetics has an effect on these things.