Pregnant women with significant depressive symptoms during pregnancy have the highest chance of delivering a baby preterm, or before 37 weeks, a new study by the Kaiser Permanente Division of Research suggests. The more severe the women’s depression is, the greater their risk of delivering preterm.

The researchers interviewed 791 pregnant Kaiser Permanente members in San Francisco city and county around their 10th week of pregnancy and noted that 41 percent of the women reported significant to severe depressive symptoms. They followed the participants for a period of two years, from October 1996 to October1998.

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 results excluded women taking anti-depressants.

The Californian-based team speculated that severe depression could prompt early birth by changing hormone levels.

There was also evidence that other factors, such as obesity and stress could increase the risk posed by depression yet further.

The study is published online in the Oxford University Press’s journal Human Reproduction on behalf of the European Society of Human Reproduction and Embryology.

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

The American Pregnancy Association estimates that one in five pregnant women may present symptoms of the depression that can determine negative consequences such as premature birth or low weight at birth.

Postpartum depression that can affect women, and less frequently men, after childbirth, has been extensively discussed by the public, “but depression during pregnancy is significantly under-recognized and under-diagnosed,” Li said. The disorder is typically associated with the changing the levels of steroid hormones in the brain.

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.