Among women with depression, 75 percent had taken antidepressants -- 77 percent before pregnancy, 67 percent during pregnancy and 82 percent after delivery, the researchers found.
Women should report any signs of depression to their doctor, Dietz said.
"There is effective treatment out there for women. You are supposed to be elated when you've had a baby. It is sometimes difficult to even bring depression up," she said. "But doctors should ask."
Dr. David L. Katz, director of Yale University School of Medicine's Prevention Research Center, said the finding that depression is very common before and after, as well as during, pregnancy is of clear importance.
"There are two potential explanations. Either the challenges of pregnancy -- from hormonal changes to psychological adjustment -- induce depression, or the medical monitoring that occurs around the time of pregnancy identifies depression that otherwise would have gone undiagnosed. Of course, both factors may be in play," he said.
There are opportunities for prevention if pregnancy is causing depression, Katz said. "If pregnancy is merely unmasking depression in the population at large, it highlights the need to screen more effectively. Finding depression is prerequisite to treating it," he said.
Dietz thinks that before a woman starts a program of antidepressants, she should discuss the risks and benefits with her doctor.
According to the March of Dimes, a woman who is depressed feels sad or "blue" for two weeks or longer. Other symptoms of depression include:
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