Increased levels of the stress hormones, glucocorticoids, can cause babies to be born smaller, Cambridge University researchers have shown.
Small foetal size is often a sign of poor foetal nutrition during pregnancy and can be a risk factor for type II diabetes and obesity later in life.
"After the terror attacks of 9/11, many mothers who were pregnant at the time gave birth to small babies" explains Prof. Abby Fowden, lead author. "This is possibly due to the prolonged level of stress such a tragic event brings."
Writing in the Journal of Physiology, Dr Owen Vaughan, explains how they used pregnant mice to study the effects of increased stress hormone levels on the growth of the foetus. Mice make a good study animal as their hormonal response to stress is thought to be very similar to that of humans.
“The mice were given corticosterone [stress hormone] in their drinking water in late pregnancy. We then used a tracer technology to measure the amount of glucose passing across the placenta to the foetus.”
Increasing stress hormone levels decreased the amount of glucose passing across the placenta to the developing foetus. Glucose, a sugar, is vital for foetal growth, so decreasing its availability led to smaller offspring.
Interestingly when stress hormone levels were increased, the mice over-ate. Contrary to what we might think this also led to decreased levels of glucose crossing the placenta, even when there was more available in the mother’s circulation. "It may be that the changes in placenta are protecting the foetus from raised levels of glucose in the mother" speculates Vaughan.
Glucocorticoid levels also increase in humans experiencing prolonged stress. Although not confirmed in this research, it is very likely that similar mechanisms occur during human pregnancy. High stress levels could be affecting the nutrients available to developing babies.
"The environmental conditions during pregnancy are influencing the growth and development of the placenta, which in turn influences the growth and development of the foetus, which has consequences for it long after it has been born," explains Fowden.
This isn't just the effect of a hard day at the office though. It is only prolonged stress that has this kind of effect. Prolonged stress is ofen unavoidable, for example a bereavement during pregnancy or a natural disaster.
There may be ways to help though, "If we can somehow alter the way [the placenta] responds to stress we might have a way of protecting placental function from changes in the maternal environment."