Women who give birth in fall and winter may be more likely to get postpartum depression than those who deliver in the spring, suggests a new study from Sweden.
Dr. Sara Sylven and her colleagues at Uppsala University say that changes in the amount of daylight during each season may affect chemical pathways in the brain related to depression.
They note that compared to most countries, Sweden has a much bigger difference in its amount of daylight in winter versus summer.
For that reason, the effect of the seasons on rates of postpartum depression might be more obvious there. But at least in similar locales, Sylven said, women who give birth in the colder, darker months should be watched more closely for symptoms of depression.
The question of whether there is a link between when a mother gives birth and her risk of depression soon after "really has flip-flopped a lot," Jennifer Jewell, who studied the association at the University of Colorado in Denver as a graduate student, told Reuters Health.
"There's a lot of mixed results out there, which I think indicates we need to know more about what we're trying to figure out," said Jewell, who was not involved in the current research.
Spurred on by that lack of concrete evidence, Sylven's team studied more than 2,000 women who gave birth at their institution over a 1-year period.
At 5 days, 6 weeks, and 6 months after giving birth, the women completed questionnaires on symptoms of depression, as well as how much social support they had and other medical and lifestyle-related topics.
Out of every 100 mothers, between 6 and 15 reported symptoms of postpartum depression, depending on how soon it was after they gave birth and in what season they gave birth.
After accounting for a variety of factors that could contribute to a new mother's stress, the authors found that women who gave birth in October through December were about twice as likely to have symptoms of postpartum depression after 6 weeks and 6 months as those who had children in April through June.
The study, published in the American Journal of Obstetrics & Gynecology, can't prove that giving birth in the fall or winter causes postpartum depression, and Sylven said the reason for the link is still unexplained.
"We guess that some hormones could differ during the year," which could be related to depression risks, Sylven told Reuters Health.
Also, she said, during the winter there's very little sunlight in Sweden, so a lack of vitamin D could be playing a role.
Depression that happens only in fall and winter - known as Seasonal Affective Disorder, or SAD - has been observed in the general population.
But Jewell isn't sure the new results would apply to women in countries that are closer to the equator and have less drastic changes in daylight from season to season.
The Swedish mothers are "at a pretty high latitude," she said. "They get some pretty different seasonal changes compared to the bulk of the United States."
Jewell said that to determine if this association exists universally, data on women from different countries could be combined if researchers figured out a way to analyze depression rates by the amount of daylight hours in the early days after birth.
Sylven noted that the effects of season on depression were most noticeable in healthy women who had no mental illnesses before giving birth.
She said the link between season and postpartum depression now has enough evidence behind it - at least in countries that have large variations in sunlight during the year - that doctors should be aware of it when treating new mothers.
"If there is such a big impact of season, we could have closer follow-up (for) the women giving birth during the autumn," Sylven said. Doctors should "think about this when (they) meet the women who will give birth in October, November, (and) December."
Jewell said that while the new research still leaves some unanswered questions, it's important to "make more women aware of (postpartum depression) and make more physicians aware of it so it's not something that's hidden and not talked about."