Teenagers are more likely to be depressed if their mothers were depressed while pregnant, according to a new study.
Mothers' depression after giving birth was also tied to their children's mental health years later, but possibly for different reasons, researchers found.
Depression during pregnancy may affect a baby through stress hormones that move across the placenta, Rebecca Pearson, from the University of Bristol in the UK, and her colleagues said.
That goes against the suggestion of some researchers that depression is only important if it continues past the end of pregnancy and impacts parenting.
"It should be treated during pregnancy, irrespective of if it continues during birth. It's as important during pregnancy," Pearson said.
She said the findings mean therapy should be made available to pregnant women with depression whenever possible. They also add another layer to the debate over the use of antidepressants in pregnancy.
The data come from a large study that began following pregnant women in England who were due to deliver in 1991 and 1992. Researchers surveyed women twice during pregnancy and twice in their baby's first year about their depression symptoms.
About 12 percent of women met the criteria for depression during pregnancy and seven percent did after birth.
Researchers then followed families over time through surveys given to the parents and children. The current study included about 4,500 children.
At age 18, eight percent of them reported symptoms of depression.
Teens were 47 percent more likely to be depressed themselves if their mothers had been depressed during pregnancy, Pearson and her colleagues reported in JAMA Psychiatry. That was still true when they untangled the effects of mothers' depression before and after birth.
The study does not prove that exposure to a depressed mother while in the womb is the cause of the teens' depression.
But if one is an effect of the other, it could be because of stress hormones such as cortisol, which are higher in depressed people and able to cross the placenta to affect the developing brain, the researchers suggested.
Or, women who develop depression before and during pregnancy may have genes that put them at greater depression risk in general, and they pass on those genes to their children, said Laura Scaramella.
She has studied the effects of maternal depression at the University of New Orleans but wasn't involved in the new research.
Teens were also depressed more often when their mothers had been depressed in the year after they were born. However, when the researchers looked at other family characteristics, they found that link only remained among children of less educated and disadvantaged mothers.
"Postpartum depression seems to have a negative impact on children's development because it affects how responsive mothers are to their babies," Scaramella told Reuters Health.
"It inhibits their ability to really attend to and respond to their baby," she said.
More educated mothers may have more support and better access to childcare, which could offset the effects of their depression on their children, Pearson's team said.
Pearson said that for any given child, the extra risk related to a mother's depression is "quite small."
But the findings do show it's important to take depression during pregnancy seriously and make sure women get help, she added.
There are still concerns about the potential harms of antidepressants to a developing baby. For example, some research suggests babies are more likely to be born early when women use the drugs during pregnancy