Could childbirth surgery be reshaping human evolution?   

Dr Ben Janaway  II  DEC 6 2016

A new study suggests that an increased number of C-sections may be reshaping human evolution. The surgery, pioneered to improve infant and mother survival during duress pregnancies. has led to an increase in larger newborns.  Increased newborn size has been linked to greater health, survival and intelligence. However, the risks posed by C-section, such as reduced immune function, means that any ‘benefit’ must be carefully considered. Some suggest a ‘runaway’ change to larger babies in future, but the complexities of evolution may render these predictions fantasy.

The study used computer modelling to predict the discrepancy between fetal dimensions and pelvic size. Conservative estimates predicted up to a 20% increase in discrepancy, meaning that 6 more infant of this size are born per 1000 births than before. The findings reflect a move toward a greater use of the surgery for a widening range of reasons, including cosmetic, older age of pregnancy and larger gestational size.  This has meant increased survival for many newborns, including those of larger cranial and body dimensions.

"The pressing question is what's going to happen in the future?"  said Dr. Philipp Mitteroecker of the Department of Theoretical Biology, Vienna. ‘I expect that this evolutionary trend will continue but perhaps only slightly and slowly’.

Evolution and infant size

Many theories have attempted to explain the curious link between maternal pelvic size and fetal dimensions. One prominent theory suggests that larger offspring are healthier and thus more likely to survive and reproduce, two cardinal prerequisites to evolutionary theory. This is backed by medical research, where larger babies are more likely to survive early childhood. Greater head circumference, an analogue for intelligence, is another trend favored as our species has matured. As we get smarter, our heads get bigger.

However, nature has imposed a limit, where an absolute size of fetus would prohibit natural birth. It is also worth noting that women with smaller pelvises may also pass on the genetic blueprint for the same, which would impose a genetically ‘locked’ limit to what child size is permitted. It is likely that, if left to nature alone, over time the presence of the ‘larger’ trait would be inhibited by the ‘smaller pelvis’. We would find a limit to traits favored by larger babies.

It seems that nature must find a medium between increase fetal size and maternal pelvic diameter, which human technology is beginning to surpass. So what of the future?

Potential influence on future evolutionary traits

Any change in external selection pressures will have a variably predictable effect on evolution. Multiple factors often work together in often unpredictable ways to shape the result of genetic lineage. Research suggests that larger infants are more likely to survive, but also that they are more susceptible to disease. It is also evident that larger children are born of parents with diabetes, which in itself is genetically linked. Furthermore, there is ample evidence to suggest that cranial size and intelligence are linked, which could suggest that larger heads in bigger infants may mean a smarter human race.

Furthermore, by delivering larger children we cannot guarantee a pelvic rim size adequate to permit their future offspring without further surgery. It may become that we reach a stage where, through selective pressures, we create a species only capable of reproducing through surgery. This is unlikely to happen at any accelerated rate and would likely be hampered by other factors, such as availability of surgery. Due to the complexity of many interwoven relationships, it is extremely difficult to make long term predictions of what C-Sections may do over our future.

What we can predict is that any changes based on C-section will preferentially affect those societies with greater access to surgery, higher rates of larger newborns (such as those with maternal diabetes) and sophisticated measures of pre-birth size estimation. It is fair to surmise that the evolutionary experiment is one of higher economically developed countries.  It may be that cost is a factor in our evolution.
Either way, it seems that humanity has found another way to alter its future.

Dr Ben Janaway MBChB  //   @drjanaway

Any opinions above are the author's alone and may not represent those of the NHS or Mind and Medicine. Any comment is based on the best available evidence at the time of writing.  All data is based on externally validated studies unless expressed otherwise. Novel data is representative of sample surveyed. Online recommendation is no substitute for seeing your own doctor and should not be taken as medical advice.

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http://www.upi.com/Health_News/2016/12/05/C-sections-may-be-causing-evolutionary-changes/4471480972832/
https://www.newscientist.com/article/2115103-are-caesareans-really-making-us-evolve-to-have-bigger-babies/
http://www.telegraph.co.uk/women/family/caesarean-myth-busterevery-woman-needs-read/