Mothers-to-be who are depressed during pregnancy have a higher risk of premature delivery, a new study suggests.

Researchers at Kaiser Permanente’s Division of Research in Oakland, California, interviewed 791 women between 6 and 18 weeks of pregnancy. About 41.2 percent were at least “significantly” depressed, and 21.7 percent had severe depression.

After taking other risk factors into account (such as a mother’s age, education level, and whether she had a preterm baby in the past), the researchers found the women with less severe depressive symptoms had a 60 percent higher risk of a premature birth compared with women without “significant” depressive symptoms. Women with severe depressive symptoms had more than twice the risk of their babies coming early.

The study’s authors concluded that treating depression might be an effective way to help prevent these premature births. Dr De-Kun Li, who led the study, said that clinicians should pay close attention to diagnose depression at an early stage and noted a worrying phenomenon: “depression during pregnancy is significantly under-recognised and under-diagnosed.” He explained that depression during early pregnancy “may interfere with the neuroendocrine pathways and subsequently placental function.”

“Preterm delivery is the leading cause of infant mortality, and yet we don't know what causes it. What we do know is that a healthy pregnancy requires a healthy placenta, and that placental function is influenced by hormones, which are in turn influenced by the brain,” Li said.

Other studies have found a link between postpartum depression and the changing levels of steroid hormones in the brain. There was also evidence that other factors, such as obesity and stress could increase the risk posed by depression yet further.

The results excluded women taking anti-depressants.