Infants born in winter months may be more susceptible to mental health problems, according to researchers at Cardiff University.
It has been long established that the winter months can be depressing for a lot of people. Now, they have been linked to stress and anxiety in people born during these seasons.
Researchers at Cardiff University found women who gave birth in autumn and winter had higher levels of the stress hormone cortisol, compared to those who gave birth in spring and summer.
The study, published in Psychoneuroendocrinology, observed over 300 pregnant women living in South Wales.
The team investigated the relationship between the seasons and symptoms of depression and anxiety. They measured levels of cortisol in the participants' saliva.
The mums giving birth in the autumn and winter were found to have 20% more salivary cortisol just before delivery than those giving birth during the rest of the year.
These findings could explain why mental health disorders are more common in people born during the winter.
Despite the team finding a link between the seasons and concentrations of salivary cortisol at term, there was no correlation between seasons and maternally-reported mental health symptoms, birth weight or placental weight.
Professor Ros John, from Cardiff University's School of Biosciences, explained: "Although maternal cortisol levels naturally rise during pregnancy, our data shows that autumn and winter babies are exposed to particularly high levels just before they are born. As higher levels of cortisol in pregnant women have previously been associated with a higher risk of children developing mental health disorders, the new findings could explain why these disorders are more common in people born during the winter months.
"They don't, however, explain the reason why women who give birth in winter or autumn have these higher levels of cortisol."