Before any of us were born, the only way we could get information from outside was via the senses of our mother. In terms of evolution, it would be greatly beneficial if we could respond to the external stimulation our mother’s sensed, since it would increase our chances of survival. This process of responding to change is called foetal programming.
Foetal, or prenatal, programming comes in various forms, from behavioural to physiological modifications – and these changes can last until adulthood. A famous example comes from a dark part of human history – the Dutch Hunger Winter.
In World War II, the Netherlands was occupied by Germany. During that period, the Nazis had cut off the food supply to a part of the Netherlands, resulting in the population starving for seven months. This tragedy, however, provided a window for scientists to look at the effects of famine on human health, especially on unborn babies.
By following up the families of the victims, scientists found children of the women who were pregnant during the time of famine were born smaller than their peers. Moreover, children were also smaller on average when growing up. This effect was also seen to be passed down to the next generation, where the children of this population were also smaller in size.
Such potent effects can also exhibit themselves in other forms. In a ten-year study, a higher lynx density is seen to correlate with a reduced birth size in snowshoe hares, with the offspring of the hare more likely to exhibit predator-avoidant behaviours. This could be explained by the increase in predation risk, leading to an increase in stress level, causing the changes observed!
In a recent study published by Dr. Zaneta Thayer and others, a similar trend can be observed across 14 different vertebrate species. In their meta-analysis of 114 results, they demonstrated the effects of prenatal exposure of maternal stress on the function of the Hypothalamic Pituitary Adrenal (HPA)-axis. The HPA-axis is the system our body uses to respond to stress, increasing our chances of survival. When exposed to a stressor, for example exam stress or the stress of predation, the HPA-axis causes stress hormones to be released, in turn affecting the body size or maturation rate of the foetus.
As 14 different species were studied, the scientists adjusted for the brain:body size and maturation rate of these animals. Their result revealed that the offspring that were exposed to higher prenatal stress level had a higher stress hormone level at birth. Amazingly, this is observed across species. This finding is significant as it suggested that this reaction to maternal stress is ancient and conserved, taking us a step forward in understanding the evolutionary origin of this response.