The rate of baby deliveries by cesarean section (C-section) varies enormously among countries, from 1 to 2 percent in sub-Saharan Africa, to more than 50 percent in Brazil. That variability is not fully understood. Cultural and socioeconomic differences are common explanations for differences among countries in the use of the procedure, but researchers have also highlighted a biological contributing factor.
Childbirth is notoriously difficult in humans, compared with other mammals, because of the tight fit between a baby’s head and shoulders and a mother’s pelvis. As fetal size increases, so does the relative mismatch between fetal head and maternal birth canal. Nutrition of many human populations has radically improved over the past several decades, leading to a gradual increase in average body height. Furthermore, babies are a generation ahead of their mothers in benefitting from improved nutrition, which compounds the fetal-mother size mismatch. Eva Zaffarini of the University of Milano-Bicocca, Italy, and Philipp Mitteroecker of the University of Vienna, Austria, hypothesized that, under these changing nutritional conditions, babies are growing a bit larger than is optimal for their mothers’ birth canals, thus contributing to increased C-section rates.
Zaffarini and Mitteroecker obtained C-section rates—from 1971 to 1996—of nearly 200 countries, using public sources such as national surveys, vital statistics, and health reports. They then used statistical models to test how C-section rates were linked to such factors as maternal body height, obesity and diabetes rates, and average age of the mother. Their model showed that, during that twenty-five-year period, an increase in average body height was an important factor linked to increased C-section rates—though it was not the only factor. Socioeconomic development and access to health care were important too.
This analysis suggests that C-section rate differences reflect an increased fetal-mother size mismatch, which is more pronounced in developing countries that are still undergoing rapid shifts in access to nutrition, compared with industrialized countries where nutritional improvements have levelled off. Given these country-specific differences, the World Health Organization’s suggestion for a global “ideal” C-section rate may need rethinking. “Without cesarian sections, there was, or in many places still is, natural selection on very large newborns,” says Mitteroecker. (Proceedings of the Royal Society B)