Researchers suggest that changes in average adult height between the 1970s and 1990s could explain about a third of the worldwide variation in Caesarean section rates, according to a study published today (February 6) in Proceedings of the Royal Society B. The researchers hypothesize that height increases may stem from improved living conditions, which could provide better nutrition and environment for the neonate and lead to an overly large fetus in relation to the mother—a situation that can prohibit vaginal delivery.

The authors argue that this points to a biological basis for the increase in caesarean deliveries and should encourage a shift in how clinicians and the World Health Organization (WHO) propose optimal C-section rates. Because the difficulty of labor could vary globally due to differences in the rates of height change, ideal C-section rates should be country-specific, the researchers add.

But experts question...

Around the world, doctors deliver babies via C-section, which can carry risks for both mother and baby. While the rates of the surgery fluctuate by country, they have been increasing, likely due to a variety of medical and cultural factors that are not yet clear. “We are all facing an epidemic of increased rates of Caesarean delivery and trying to reduce these trends,” says Eyal Sheiner, an obstetrician at Ben-Gurion University of the Negev in Israel who did not participate in the study.

The authors of the new study acknowledge the influence of cultural factors, but propose a biological explanation—increases in body height—for the increasing rates of C-sections. A couple of years ago, study coauthor Philipp Mitteroecker, a theoretical biologist at the University of Vienna, encountered two maps: one of C-section rates and one marking body height increases—which happened worldwide through the 1960s, but have varied by country since then. Struck by overlap between the two, Mitteroecker and graduate student Eva Zaffarini used mathematical modeling to assess the possible association between height change and delivery method.

They looked at changes in average body height from 1971 to the 1996—the time during which most people who have given birth within the past decade or so were born—and found a strong correlation to C-section rates from 2005 to 2017. For instance, in China, body height increased about a tenth of a centimeter per year over the time period, and the country had a C-section rate of about 35 percent—well over the WHO’s recommended 10 to 15 percent. In Cameroon, on the other hand, average height fell by more than a tenth of a centimeter per year and had a C-section rate around 5 percent.

After accounting for possible confounding factors, including socioeconomic development, access to and quality of healthcare, and maternal age, the authors found that a third of the international variation in Caesarean rates can be accounted for by changes in height within that time period.

The findings “should be the start of a paradigm shift in the way people see global variation in Caesarean section rates and [how] the difficulty of childbirth is affected by environmental and biological factors,” Mitteroecker tells The Scientist. “It’s clear that there are many cultural factors influencing obstetric practices, but there really seem to be biological differences in the average fit between the fetus and the birth canal. This should be taken into account when devising optimal C-section rates.”

Sheiner points out that populations’ changing body size is just one factor that could be influencing the trend toward high C-section rates, and that the decision about how to deliver is complicated and personal. The study is interesting, but just another item in a growing list of factors to consider when making recommendations about delivery methods, he says.

Tim Cole, a medical statistician at University College London who was not involved in the study, warns that gains in adult height may not be the most relevant factor. Obesity has also been on the rise in both kids and adults of developed countries—a factor he says the authors did not adequately address. While they adjusted for maternal obesity in their analyses, “they didn’t adjust for the change in maternal obesity,” he says. “If they’re focusing on height change, logic suggests that they should check for weight change as well.” Cole suspects that the authors identified an indirect association and that the real driver of the correlation between C-section rates and changes in average height is changes in weight.

“It’s hard to pull out one factor in an ecological study like this because there are so many other things that are going on aside from the one factor that you’re interested in,” cautions Emily Oken, a physician and public health researcher at Harvard Medical school who was not involved in the work. In fact, a 2010 study from her group showed that, from 1990 to 2005, birth weight actually decreased in the US, while rates of C-sections increased.

Clearly, bigger babies are not the only possible reason for the rise of Caesarean deliveries over the same time period, says Oken. It’s not appropriate to make recommendations for changing practices around Caesareans based on an indirect connection, she adds; more research and data are necessary to figure out what’s really going on. 

E. Zaffarini and P. Mitteroecker, “Secular changes in body height predict global rates of Caesarean section,” Proc Roy Soc B, doi:10.1098/rspb.2018.2425, 2019.

Clarification (February 8): This story has been updated to avoid any implication that the study found a causal relationship between C-section rates and changes in average height. The findings were correlative in nature. The Scientist regrets any confusion.

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