A hormone that prepares pregnant women for the strain of childbirth could help explain why some women suffer postpartum depression. Researchers from the University of California, Irvine, found in a study that 12 out of 16 women who had postpartum depression also had high levels of corticotropin-releasing hormone circulating in the placenta midway through pregnancy.

The hormone, known as CRH, normally is produced in very small amounts by the hypothalamus near the brain in response to stress. The placenta in pregnant women, meanwhile, produces 100 times more CRH than the hypothalamus.

Postpartum depression strikes those who experience the biggest changes in hormone levels, said researchers, who noted 20% of women are affected by postpartum depression four to six weeks after childbirth, and 7% of new mothers suffer from major depression.