Depression during pregnancy was found to double the chance of a woman to deliver her baby before term or at less than 37 completed weeks of gestation. The study conducted by the Kaiser Permanente Division of Research was published this week in the journal Human Reproduction.

It involved 791 women during pregnancy, two-fifths of them reporting significant depressed feelings. Overall, women with significant depressive symptoms were found to be 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.

The researchers speculated that severe depression could prompt early birth by changing hormone levels.

“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. De-Kun Li, lead author of the study said.

The bad part is that depression during pregnancy is often under-diagnosed both by women and by obstetricians, Dr. Li added. According to the American Pregnancy Association, one in five pregnant women may present symptoms of depression, which can determine negative consequences such as premature birth or low weight at birth.

Preterm delivery is the leading cause of infant mortality although researchers have not found what exactly causes the death. According to the latest statistics released by the federal Centers for Disease Control and Prevention, deaths related to preterm birth have increased. More than half of infant deaths in the U.S. occur in babies born extremely early – before 32 weeks gestation.