Regular readers may have noticed that I haven’t been posting as often as before. I have been somewhat distracted recently… But why not combine what is happening with this blog. So in a new section I will be talking about science around babies (yes this is a strange kind of announcement 😉 👶).
When scientists talk about pregnancies, it’s not normal baby talk. Non-scientists would not believe some of the topic pregnant scientists discuss. So, all for over-sharing, I thought I share the discussion points with you over a series of blog posts.
One conversation I recently had at work revolved around the observation that more and more women seem to have c-sections and that babies, at least subjectively, are getting bigger and bigger. Morbid as we apparently are, we realised that quite a few of our friends would have died 220 years ago before the first successful c-section was performed in 1794. And while the number of c-sections has, in part due to planned, not-medically necessary ones, sky-rocketed in recent years (more than 25% of births in the UK, Ireland, USA and Canada are c-sections) – relieving the evolutionary pressure on head and pelvis size may have long-term effects for child birth.
In this context I immediately thought of French Bulldogs. People have (mis-)bred that dog so badly that only very few litters are born normally. Most have to be brought into this world via c-section, because their heads are simply too big. They do not fit through the pelvis of the pregnant dog anymore. Before c-sections, big-headed babies (and moms with small pelvises) wouldn’t survive. That way head-size was kept “manageable”. Now, with improved methods, luckily, more and more babies survive birth, even if their heads are “too” big. Does that mean that over the next hundreds of years, without evolutionary pressure, heads and babies will get even bigger until most babies need to be born via c-sections?
Looking for scientific evidence for our discussion I found the following letter from Canadian obstetrician-gynaecologist Dr. Joseph Walsh supporting our string of thoughts. “Prior to the availability of cesarean section, women with a small pelvis, and babies of large birthweight, were at risk of significant morbidity and mortality. With cesarean section, selection against a small pelvis or large birthweight has been all but eliminated,” Walsh writes. “The maternal pelvis can get smaller over time, and fetal birthweight can get greater over time, because there is now nothing to limit these changes.”
A study published in PNAS in 2016, used mathematical modelling to look at obstructed labour and made some interesting points. For one, it reviewed the literature as to why evolution has not limited brain size or increased pelvic dimensions to limited obstructed labour, which has a strikingly high incidence of 3-6% in humans. Both large brain size in babies and a narrow pelvis have evolutionary fitness advantages. Birth weight, which correlates with brain size at birth, is strongly associated with infant survival; while a narrow pelvis is believed to have biomechanical advantages for bipedal locomotion (= walking on two legs instead of four). There is a trade of between brain size and pelvis dimension, where too big beads and too narrow pelvis lead to a complete loss of fitness (= death) but survival fitness is at its greatest right before the cut off.
Applying their model to the increased use of c-sections and therefore the loss of evolutionary pressure on both brain and pelvis size, a 9-20% increase in the incidence of obstructed birth was predicted. Which means babies heads will likely get bigger and pelvis’s will get smaller given their evolutionary advantages stated above and the lack of pressure to not get too extreme. Hence the need for medically-relevant c-sections will further increase. A spiral of sorts.