Women who give birth in winter or spring are less likely to suffer the "baby blues" than at other times of the year, a study has shown.
Having a longer pregnancy also reduced the risk of suffering from post-natal depression, doctors in the US found.
But not having an epidural during delivery was found to increase the risk, according to researchers.
A team at Brigham & Women's Hospital in Boston reviewed the medical records of 20,169 women who delivered babies between June 2015 and August 2017. Of the total, 817 (4.1%) suffered from post-natal depression.
Symptoms include sadness, restlessness, and lack of concentration.
The condition typically arises from a combination of hormonal changes, psychological adjustments to motherhood and fatigue.
Dr Jie Zhou said: "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."
Why giving birth in winter or spring should have a positive effect is not known but could be linked to the "seasonal enjoyment of indoor activities mothers experience with newborns", said the researchers.
Caucasian women were less likely to experience post-natal depression than women of other races, the research showed. Delivery mode had no effect.
The findings were presented at the Anesthesiology 2017 annual meeting in Boston.