Studies have shown that it is not uncommon for women to experience depressive symptoms or a depressive disorder during pregnancy. While reports are few and vary, it can be estimated that 25-35 % of women may report depressive symptoms during pregnancy, yet less than 20 % would meet criteria for a minor or major depressive disorder. However, while the rate of depressive disorders may be lower during pregnancy than during the postpartum period, it remains substantial; some researchers have found that it is as high as in nonpregnant groups.

The clinical course of unipolar depression varies throughout pregnancy. Most researchers have found a peak during the first trimester with improvement during the second trimester, and an increased rate again during the third trimester.

In addition, several research groups have documented that depressive symptoms during pregnancy are the most robust prediction of postpartum depression.