Women with symptoms of depression may have nearly twice the risk of preterm delivery compared with women with no symptoms, a study suggests.

Women without depression had a 4.1 per cent risk of giving birth before 37 weeks, compared with 5.8 per cent among those with mild depression and 9.3 per cent for severe depression, researchers said in Thursday's online issue of the journal Human Reproduction.

Postpartum depression is well documented, but depression during pregnancy is significantly under-recognized and under diagnosed, said the study's lead author, Dr. De-Kun Li, a reproductive and perinatal epidemiologist at Kaiser Permanente's Division of Research in Oakland, Calif.

"Clinicians should pay close attention to depression during pregnancy to catch it early," Li said.

"If prenatal depression is indeed as prevalent as reported in this and other studies and doubles the risk of preterm delivery, then bringing depression to the forefront of prenatal care could lead to a significant reduction of preterm deliveries."

A healthy pregnancy requires a healthy placenta, which depends on hormones influenced by the brain, Li said.

It's thought that depression during early pregnancy may interfere with the release of endocrine hormone, which affects function in the placenta, he added.

In the study, researchers looked at 791 women in and around San Francisco from October 1996 to October 1998.

When the women were interviewed around their 10th week of pregnancy, 41 per cent reported significant or severe depressive symptoms. More than 98 per cent of the women did not use antidepressants.

"Our findings show that pregnant women with depressive symptoms are at increased risk of preterm delivery and, in addition, provide preliminary evidence that social and reproductive risk factors as well as obesity and stressful events may exacerbate the effect," the study's authors concluded.