Fully one-third of the global increase in caesarean section rates over the past few decades can be attributed to improving parental height and foetal size, research shows.
Rates of C-sections have sky-rocketed in some countries since the mid-twentieth century.
As many as 5.8% of babies need to be delivered this way because they are, simply, too big to fit down a mother’s birth canal.
Many more mothers opt for C-section delivery for a wide range of reasons. For a large number, the baby is considered to be large enough to create significant pain during vaginal delivery. For others, maternal obesity or ill health may prompt doctors to recommend a C-section. Psychological and economic factors, as well as access to healthcare services, sometimes also play a role.
Writing in the journal Proceedings of the Royal Society B, researchers Eva Zaffarini from the University of Vienna in Austria and Philipp Mitteroecker from Italy’s University of Milano-Bicocca report that as a result of all these factors C-section rates vary enormously between countries.
In sub-Saharan African countries, for instance, the rate can be as low as 1%, while in Egypt, Turkey and Brazil it is the preferred method for as many as 50% of births.
“Despite global differences in obstetric traditions, healthcare system and nutrition, this massive international variation of C-section rate remains puzzling,” they write. “It is usually attributed to socio-economic, legal and cultural heterogeneity.”
Zaffarini and Mitteroecker propose an additional perspective: improvements in intergenerational health and nutrition.
Since the mid-twentieth century, they report, and especially in the period lasting from 1970 to the end of the century, in most countries, particularly the wealthier ones, average birth weight and adult body height have increased year-on-year.
These two measures, however, while both going up, have not been in proportionate lock-step.
In this progression, the researchers point out, any foetus is a generation ahead of its mother and thus “is likely to have experienced — on average — a more beneficial environment than the mother did”.
And while that might be heart-warming in one way, it’s problematic in another.
The result, say the researchers, is that on average, by the time of delivery, a baby’s head is likely to be larger than the optimal size suitable for vaginal delivery.
“Despite a certain degree of adaptation of foetal growth to the intrauterine environment, the rapid intergenerational improvement of living conditions, and thus also foetal provisioning, may challenge childbirth,” they conclude.
Indeed, they show that one-third of the global variance in C-section rates between the 1970s and the 1990s can be explained by trends in adult body height. Using a structural equation model, they reveal that while factors such as access to healthcare certainly had an influence “the strongest direct effect on C-section rate was body height change”.
Zaffarini and Mitteroecker report that in many parts of the world the disparity between foetal growth rate and adult body height has eased somewhat, plateauing in some countries and even going backwards in others. (Needless to say, this not in most cases good news.)
Nevertheless, a degree of intergenerational difference remains, expressed differently in different countries.
This leads the researchers to suggest that the World Health Organisation’s published “ideal rate” for C-sections needs to be urgently reviewed. The WHO posits a rate of between 10% and 15% of births as desirable.
Zaffarini and Mitteroecker call for “a more differential view” that shifts from purely cultural values to “a combined biocultural perspective” that takes into account changes to human anatomy and physiology as well as other factors.
Andrew Masterson is a former editor of Cosmos.
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