Around 10 percent of women are affected by the so-called “baby blues” after giving birth. Now, new research shows that women who give birth in winter or spring are less likely to get postpartum depression (PPD) than those who have babies in the summer or fall.

"We wanted to find out whether there are certain factors influencing the risk of developing postpartum depression that may be avoided to improve women's health both physically and mentally," said lead study author Dr Jie Zhou of Brigham and Women's Hospital in Boston in a statement.

PPD is usually caused by a combination of hormonal changes, psychological adjustments to motherhood, and fatigue. The symptoms include things like sadness, restlessness, anxiety, sleeping problems, and decreased concentration. If untreated, PPD can be distressing for everyone involved and can affect how well the mother bonds with her new baby. Interestingly, as many as 1 in 10 new dads get depression too.

The researchers reviewed the medical records of more than 20,000 women who had babies from June 2015 through August 2017. Just over 4 percent of them suffered from PPD. The findings were announced at the American Society of Anesthesiologists 2017 Anesthesiology annual meeting.

It is unclear why women giving birth in the winter and spring months are less likely to get PPD, but there are various possibilities. For example, women with new babies tend to have to stay inside. In winter this is less of an issue, but in summer, when people are out and about more, it can leave mothers feeling trapped and isolated from their friends.

It could also be related to vitamin D – which comes from sunlight – being depleted over winter so it is particularly lacking by summer and fall. Low vitamin D levels have been linked to PPD in a number of previous studies.

Nevertheless, the researchers found that it wasn’t just timing that affected a woman’s risk of PPD. In fact, they found that heavier women, with higher BMIs, were more prone. This might be related to higher risk of complications and more follow up medical checks after birth.

Women who didn’t have pain relief such as epidurals were more also likely to suffer from PPD, although this might just be an effect of the trauma of experiencing such terrible pain.

Meanwhile, Caucasian women were found to be less likely to get PPD than women of other races, but this “may be due to differences in socioeconomic status among these ethnicities,” noted Zhou.

Higher gestational age was found to actually decrease the likelihood of depression – essentially the further along in the pregnancy you give birth, the better. "It is expected that the mother will do better and be less mentally stressed when delivering a mature, healthy baby," explained Zhou.