Even brief bouts of severe depression among mothers double the risk of depression in their offspring, a study finds. However, short periods of mild depression do not seem to influence a child's risk of the disorder.

It has long been known that maternal depression puts children at risk for depression and other mental disorders. Researchers have also attempted to untangle how severity, chronicity and timing of the disorder affect offspring. UCLA psychologist Constance Hammen, who tracked more than 800 women and their kids, found that 20 percent of children born to depressed mothers had depression by age 10.

By contrast, depression occurred in only 10 percent of kids under age 10 born to women who had never been depressed. Children of depressed moms also had double the anxiety disorders.

Severe maternal depression, even of one or two months, was the strongest predictor of childhood depression. But, mild depression for 12 months or longer also increased the risk of depression among offspring. The age of a child at the time the mother became depressed did not affect risk.