In the study, researchers found that pregnant women with symptoms of depression have an increased risk of preterm delivery, and that the risk grows with the severity of the depressive symptoms.
These findings also provide preliminary evidence that social and reproductive risk factors, obesity, and stressful events may exacerbate the depression-preterm delivery link, according to the researchers.
Because the majority of the women in the study did not use anti-depressants, the study provides a clear look at the link between depression and preterm delivery.
The study examined 791 pregnant Kaiser Permanente members in San Francisco city and county from October 1996 through October 1998.
Researchers interviewed the women around their 10th week of pregnancy and found that 41 percent of the women reported significant or severe depressive symptoms.
The women with less severe depressive symptoms had a 60 percent higher risk of preterm delivery, defined as delivery at less than 37 completed weeks of gestation, compared with women without significant depressive symptoms, and the women with severe depressive symptoms had more than twice the risk.
"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," said lead author Dr. De-Kun Li, a reproductive and perinatal epidemiologist at Kaiser Permanente's Division of Research in Oakland.
"This study adds to emerging evidence that depression during early pregnancy may interfere with the neuroendocrine pathways and subsequently placental function. The placenta and neuroendocrine functions play an important role in maintaining the health of a pregnancy and determining the onset of labour," Li said.
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.