The rate of Caesarean sections is increasing, as more women have to go under the knife to give birth. Obviously, the medical benefits are enormous, preventing potentially fatal situations for both mother and child. But a new piece of research suggests, perhaps quite controversially, that the procedure may also be altering human evolution.
The researchers of the study state that the estimated number of cases in which babies cannot be born vaginally have increased from around 30 in 1,000 births in the 1960s to 36 in 1,000 in the present day. This equates to a 20 percent rise in obstructed childbirths, something that they postulate could be altering the evolution of humans.
There is a selective pressure for babies to be born larger, as bigger babies are more likely to survive. But this evolutionary pressure is countered by another that selects for women to have a narrow pelvis, as it is thought to help women walk upright as well as prevent the chance of a premature birth. These two opposing factors mean that the size of a baby’s head closely fits that of a woman’s pelvis.
However, the researchers claim that with an increase in the rate of caesareans, the balance between these forces is now shifting. In the past, if a baby’s head was too large or a woman’s pelvis too narrow, the baby would get stuck in the pelvis and usually kill the child and potentially the mother, thus preventing the genes from being passed on. However, this may no longer be the case.
“Without modern medical intervention such problems often were lethal and this is, from an evolutionary perspective, selection,” explains the University of Vienna’s Dr Philipp Mitteroecker, who co-authored the study published in PNAS. “Women with a very narrow pelvis would not have survived birth 100 years ago. They do now and pass on their genes encoding for a narrow pelvis to their daughters.”
This could, in theory at least, lead to evolutionary changes. But this isn’t necessarily what is being seen. The scientists haven’t actually provided any evidence to back up their postulation, and instead carried out their work using statistical analysis based on data collected from the World Health Organization.
It is true that the rate of caesareans is increasing, especially across the Western world, but there are many possible reasons for this and these factors are difficult to separate. For example, doctors will recommend a C-section to older mothers, due to an increased risk of vaginal tearing, and with the average age of women giving birth on the rise, this could help explain the increase in caesareans. Other possible factors include diabetes and obesity.