Depression during pregnancy raises considerably a woman’s chance to deliver her baby before term, according to a study published Thursday in the journal Human Reproduction. More exactly, women experiencing severe depression during pregnancy have twice the risk of their babies coming early.

The bad news is that “depression during pregnancy is really under-diagnosed, both by women and by obstetricians,” said lead author De-Kun Li, a reproductive and perinatal epidemiologist at Kaiser Permanente’s Division of Research in Oakland.

According to the March of Dimes, about 12.5 percent of babies born in the United States each year are preterm. Preterm delivery is defined as delivery at less than 37 completed weeks of gestation. Generally, babies born before term have few problems, but early delivery remains the leading cause of infant mortality although no clear reason was given yet.

“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,” Dr. Li said.

For the study, Dr. Li and colleagues monitored 791 women during pregnancy. Two fifths of them reported significant depressed feelings. Overall, women with significant depressive symptoms were nearly twice as likely to deliver a baby preterm compared to those without significant depressive symptoms.

Actually, the more severe the women’s depression, the greater their risk of delivering preterm, the study found. Less-educated women or women with past fertility problems, at least two previous pregnancies, or a history of stressful life events were most at risk for a depression-related preterm birth.