Caesarean sections have helped overcome many a life-threatening childbirth since they became more commonplace in the 1950s, but what of their influence on human evolution? A new study suggests that the growing number of mothers giving birth in this way is leading to a higher number of bigger baby heads making it into the world, whose genes have been passed on to result in a "strikingly high" incidence of obstructed labor today.
The concept of a caesarean section has been around for centuries, indeed some speculate the Julius Caesar was born in this way and gave rise to its name. But medical advances made it a whole lot safer following World War II, and the procedure has been used routinely in industrialized countries since around that time.
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A c-section might be opted for when the baby is too large to fit through the mother's pelvis, circumstances where a natural birth would place the baby and mother's life in danger. This is a relatively common occurrence compared to childbirth in other primates, thanks to the disproportionately large fetus compared to the human mother's pelvic canal.
It doesn't seem all that surprising that the higher rates of babies delivered through c-section will result in a higher amount of bigger babies in the world – it is, after all, the procedure's desired outcome. But in an effort to quantify its consequences and eye any emerging evolutionary trends, an international team of scientists carried out an analysis of birth data over the last half a century.
It found that the global rate of fetopelvic disproportion (when the baby's head is too big to fit through the pelvic canal), has risen by between 10 and 20 percent, from 3 percent of births in the 1960s to 3.3-3.6 percent today, which the researchers describe as strikingly high.
"We predict that the regular use of Caesarean sections throughout the last decades has led to an evolutionary increase of fetopelvic disproportion rates by 10 to 20 percent," the researchers write, though they do point to other possible explanations. These include substantial changes in nutrition over the last century, along with migrating mothers with narrower pelvises raising fetuses on high-protein western diets.
The research was published in the journal Proceedings of the National Academy of Sciences.