Women who experience postnatal depression have fewer children than women who don’t, and the worse the depression, the less likely they are to carry on having children. This may seem unsurprising, but until now no one had ever checked. How many children women have is important, not just to the women themselves but also in terms of population structure; the fewer children born, the slower population growth and, past a certain point, the quicker population ageing. Postnatal depression is very common, yet prevention and treatment are poorly funded – the prospect of slowing population ageing may just provide a much needed incentive to address this.

It is often reported that postnatal depression, i.e. depression experienced in the first year after giving birth and usually within 6 weeks, has a prevalence of 13%. However, it is also estimated that more than 50% of cases go undiagnosed in the UK, and in the US as many as 63% of women experience symptoms of depression after giving birth. What is for sure is that not enough is done to prevent it and not enough is known about the consequences of postnatal depression for those women who do experience it. Given so many women undergo dramatic reductions in their emotional wellbeing after childbirth, it should be surprising (although perhaps it isn’t given the dearth of scientific research into female experience) that researchers had never asked what this does to the likelihood a woman will go on to have another child. In fact, without such information having been documented, a prominent evolutionary hypothesis was developed suggesting that postnatal depression is an adaptive mechanism evolved to enable women to save or gain additional resources, thus benefiting females.

Very little in the way of existing data was available to address this knowledge gap, so as part of my PhD under the supervision of Dr Sarah E. Johns and Dr Oskar Burger at the University of Kent I gathered the reproductive histories of around 300 (what evolutionary anthropologists would call) WEIRD postmenopausal women – WEIRD standing for Western, Educated, Industrialised, Rich, and Democratic. I found that women who did not have postnatal depression at their first birth had an average of 2.30 children, while women who did have postnatal depression had 1.96 – this might not look like a big difference, but it is statistically significant and when one considers women must have 2.07 children or more for the population not to age, it starts to look like a lot.

The impact postnatal depression has on female reproductive behaviour seems to be cumulative, either influencing a woman’s physical or emotional capacity to have increasing numbers of children. As the severity of postnatal depression increased in the period after a woman’s first birth, the odds of her having a third birth (rather than a second) decreased (Figure 1). Having two bouts of postnatal depression also made a striking difference to a woman’s childbearing, such women were 76% less likely to have a third child compared to those who had no postnatal depression – this is particularly important in terms of impact at a population level, because women who have postnatal depression at their first birth have a 50% chance of having it at their second as well.

There are a variety of factors well known for their influence on female childbearing. Particularly salient in contemporary WEIRD populations, where childbearing is often ‘delayed’, is a woman’s age; this is because fertility rapidly declines from the late-20’s until it reaches effectively zero in the mid-late 40’s (technological assistance aside). I found postnatal depression has a comparable effect – in terms of the reduction in the chances of having a third birth after the second, every increase of a point on the Edinburgh Postnatal Depression Scale was similar to increasing in age by a year. Having two bouts of postnatal depression was equivalent to experiencing major birth complications.

Why might this be? It is possible that women are simply put off having children by postnatal depression, and this is undoubtedly the case for some women. However, the fact that symptom severity at the first birth decreased the odds of women having a third birth but not a second, suggests there are long term physiological, as well as emotional, effects. Postnatal depression is known to be linked to changes in reproductive hormones and women with a lifetime history of depression are at risk of earlier menopause. Not only are women more likely to have postnatal depression if they’ve already had it, they are more likely to have depression at other points in the future too; thus, women who experience postnatal depression may be physically less able to have more children.

What good does knowing postnatal depression is linked to lower levels of childbearing do? For starters, it sheds light on a previously undocumented consequence of female experience, which is important in its own right. Second, it contributes to ongoing theoretical debates regarding the evolutionary origins of postnatal depression, and depression more generally (and for those interested you can find the discussion generated within the field by these results here and here). Finally, as hinted at the top, on a practical level these results should interest governments struggling to deal with the massive financial implications of population ageing. Population ageing occurs when individuals, on average, do not have enough children to replace themselves in the population, leading to population structures with more old people than there are young people to support them. This presents major issues for health and social security systems, amongst other things, leading governments to seek ways to raise fertility levels. Typically this involves improving practical things like parental leave – while this is obviously to be encouraged, the emotional lives of parents go unconsidered. At the same time, investment in mental health is widely sub-optimum. It was recently calculated that an extra £400 per birth in the UK would elevate perinatal mental health services to the level needed to effectively deal with the mental health needs of all mothers, spending that seems unlikely in the current political climate purely for the sake of mothers and their children (if only this were enough!). However, recognition that £400 per birth could in fact lead to more births may tip the scales, and without wishing to sound pronatalist, anything that encourages spending on maternal mental health can only be a good thing.

Open access to the award winning original research article on which this blog was based can be found below:

Myers, S., Burger, O., and Johns, S.E. (2016). Postnatal depression and reproductive success in modern, low-fertility contexts. Evolution, Medicine, & Public Health 2016(1): 71-84.