The discovery could help identify and treat women so affected long before the onset of symptoms, said Ilona Yim, assistant professor of psychology and social behavior.
Normally secreted in very small amounts by the hypothalamus, the hormone regulates the body's response to stress, Yim said. During pregnancy, large amounts are produced in the placenta and are association with delivery.
"The hormone we studied plays an important part in pregnancy and has been linked to depression," Yim said. "Many factors may cause some women's bodies to produce more of this hormone during pregnancy. Evidence suggests that stress early in pregnancy could play a role."
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During a study funded by the National Institute of Child Health and Human Development, researchers took blood samples from 100 pregnant women and assessed symptoms of depression throughout pregnancy, then again nine weeks after delivery, Yim said.
Of those, 16 developed postpartum depression symptoms during follow-up visits, and three quarters of those women could have been identified in mid-pregnancy, solely based on hormone levels, Yim said.
The study appears in the February issue of Archives of General Psychiatry.