The massive global variation in caesarean-section (C-section) rate is usually attributed to socio-economic, medical and cultural heterogeneity. Here, we show that a third of the global variance in current national C-section rate can be explained by the trends of adult body height from the 1970s to the 1990s. In many countries, living conditions have continually improved during the last century, which has led to an increase in both fetal and adult average body size. As the fetus is one generation ahead of the mother, the fetus is likely to experience better environmental conditions during development than the mother did, causing a disproportionately large fetus and an increased risk of obstructed labour. A structural equation model revealed that socio-economic development and access to healthcare affect C-section rate via multiple causal pathways, but the strongest direct effect on C-section rate was body height change. These results indicate that the historical trajectory of socio-economic development affects-via its influence on pre- and postnatal growth-the intergenerational relationship between maternal and fetal dimensions and thus the difficulty of labour. This sheds new light on historic and prehistoric transitions of childbirth and questions the World Health Organization (WHO) suggestion for a global 'ideal' C-section rate.
Keywords: body height; childbirth; dystocia; human evolution; obstetrical dilemma.