New York, Sep 28 (IANS): Fewer daylight hours during the third trimester of pregnancy may add to a woman's risk of developing postpartum depression once the baby is born, suggests new research.

The findings, published in the Journal of Behavioural Medicine, suggest that clinicians need to encourage at-risk women to increase their exposure to natural daylight and vitamin D.

"Women should be encouraged to get frequent exposure to daylight throughout their pregnancies to enhance their vitamin D levels," said lead study author Deepika Goyal of San Jose State University in the US.

Clinicians should also advise their patients to get more exercise outdoors when weather and safety permit, she added.

In this study, Goyal and her colleagues at the University of California San Francisco analysed available information from 293 women who participated in one of two randomised controlled clinical trials about sleep before and after pregnancy.

The participants were all first-time mothers from the US state of California.

Data included the amount of daylight during the final trimester of their pregnancy, along with information about known risk factors such as a history of depression, the woman's age, her socioeconomic status and how much she slept.

Overall, the participants had a 30 per cent risk of depression. The analysis suggested that the number of daylight hours a woman was exposed to during her final month of pregnancy and just after birth had a major influence on the likelihood that she developed depressive symptoms.

The lowest risk for depression (26 per cent) occurred among women whose final trimester coincided with seasons with longer daylight hours.

Depression scores were the highest (35 per cent) among women whose final trimester coincided with "short" days and the symptoms continued to be more severe following the birth of their babies in this group of women, the study said.