Can we put preterm birth in evolutionary context and maybe understand why there is a relatively high percentage of preterm births present in populations around the world?
Part of my studies, apart from hands-on experience regarding women’s health, is also focused on theory, especially evolutionary theory, as the evolutionary perspective becomes more and more important in medicine due to the lack of newly developed treatments and approaches.
Evolutionary medicine
Evolutionary medicine tries to explain certain patterns (read: diagnoses) in an evolutionary context. For example, people’s lifespan is much longer than it used to be. People nowadays live up to 80, some even longer, while before medical development, people were living up to 40, 50 years. Therefore, gene predispositions that served our ancestors were mostly there to allow people to have, early in life, as many healthy children as possible, while long-term survival was limited up to 50 years or so. Evolutionary medicine tries to explain exactly these genes that once served us well in building healthy offspring, but whose potential dysfunctions wouldn’t matter much since people would die soon after their fertile part of life ended.
But since we live longer and give birth later in life, these genes stay, and their opposite and potentially dangerous effects stay, too. The most famous gene is the BRCA gene. If mutated, it can cause breast cancer, but once, this mutation allowed women to have more children and larger families. And since our health system has improved in cancer treatment, women with these mutations can have children and pass on these genes to future generations as well. These are exactly the topics that evolutionary medicine tries to understand, in order to help build better, maybe even gene-targeted, solutions for some diseases.
What is pre term birth
In this context, the topic of preterm birth can also be discussed. Any birth that happens before the 37th week of pregnancy is considered preterm, and nearly 50% of infant deaths are due to prematurity.
To understand what is really going on when evolution jumps into the explanation, it’s good to mention that, first of all, prematurity is often caused by other intrauterine problems such as preeclampsia (low implantation success of the placenta), intrauterine infections, or some other complication. But to explain these underlying problems, evolution might also be applied.
Theories
There is a theory called the PAR hypothesis (Predictive Adaptive Response), which states that intrauterine changes are caused by the existing environment, so that the child can adapt to what’s expected outside. There are many examples of these adaptations in the animal world, which cause certain traits to develop in difficult conditions, traits that wouldn’t usually be there. These traits often come with a cost, such as reduced reproductive success or later life challenges. But it’s not just animals, there is evidence in humans, too.
Low birth weight might be a perfect example of this hypothesis. In some countries, low birth weight occurs when there is a lack of food, the tissue development is reduced so the child can survive in the womb until birth. Physiological changes such as better fat storage, energy usage, and altered insulin activity are developed so the child can survive years of hunger. But also, some scientists and their work, ike that of David Barker and colleagues, who studied men born in Hertfordshire, England (1911–1930), developed the DOHaD hypothesis (Developmental Origins of Health and Disease). This hypothesis states that if a child is born with low weight (<2500g), they have a higher chance of developing diseases like stroke and heart disease. What they concluded is that not only genes or lifestyle later in life shape health, but also early development, which still happens in utero. Therefore, a mother’s lifestyle matters a lot when it comes to the child’s risk for chronic illnesses later in life.
So, going back to the topic: is it possible to say that preterm birth can be explained by the PAR hypothesis? It’s definitely something to consider.
Look into it
What is interesting is that different species also have different gestation lengths, and what’s more, they also differ in their newborn survival strategies. Some animals are born very developed, like foals or antelopes, and can stand and move almost immediately after birth. Others, like human babies, are born less developed and fully depend on caregivers. This is tied to what’s called the altricial–precocial spectrum, a term that describes how mature newborns are. Humans are on the more altricial side: we give birth to underdeveloped babies.
This might not be random. One theory suggests that human evolution pushed for bigger brains, but a bigger head means it’s harder to pass through the birth canal. To avoid risking the mother’s life during childbirth, evolution likely favored earlier births, thus giving birth to babies that are still developing. This is known as the obstetric dilemma. As human brains grew bigger, birth had to happen earlier.
This means that we might be giving birth earlier than the body would naturally prefer, just to make sure that both the baby and mother survive. So, in a way, human birth is already sort of premature.
If that’s the case, then actual preterm birth (as we medically define it) might be an exaggeration or disruption of a process that was already a compromise. Evolution might have set up a very delicate timing for birth, and anything from stress, infections, poor nutrition, or lack of support can tip it too far.
Also, looking into stress and social factors, there’s another angle. Some studies show that women under chronic stress, whether social, financial, or emotional, are at higher risk for preterm birth. This might be linked to biological stress responses, like cortisol levels, but also to the way the body tries to prepare the child for a potentially harsh world outside.
In other words, the fetus is getting the signal: “It’s rough out there, you better come out now and start adapting.” But coming out too early comes at a cost: the baby might not be ready for life outside the womb.
Possible conclusion
So maybe preterm birth is not just a random accident. Maybe it’s biology’s way of trying to help. But evolution is not perfect. Sometimes it overreacts. Sometimes the environment has changed too much, too fast for our biology to keep up. And that’s where medicine steps in.
With better prenatal care, early screenings, understanding stress, and maybe even gene research, we can try to help this process go as naturally and safely as possible.
Preterm birth is a complex, multifactorial outcome, part evolutionary strategy, part biological limitation, and part modern lifestyle effect. And looking at it from all those angles can only help us understand it better.
Reference:
Williams TC, Drake AJ. 2019 Preterm birth in evolutionary context: a predictive adaptive response? Phil. Trans. R. Soc. B 374: 20180121. http://dx.doi.org/10.1098/rstb.2018.0121