An American study has suggested women with major depression risk a recurrence of their condition if they stop taking their drugs during pregnancy.
Women and doctors have to decide whether to continue with medication
It had been thought the hormone changes which occur during pregnancy gave a "protective" effect against depression.
But the Journal of the American Medical Association study of 201 pregnant women found this was not true, and women who came off their medication relapsed.
A UK expert agreed it was better for pregnant women to stay on their drugs.
Dr Veronica O'Keane said the only exception was paroxetine, which a study last year suggested could be linked to birth defects.
History of depression
In this latest study, researchers from Massachusetts General Hospital and Harvard Medical School, Boston studied 201 pregnant women between 1999 and 2003, who were treated at centres which focussed on the care of psychiatric illness during pregnancy.
All the participants had a history of major depression prior to pregnancy, were less than 16 weeks pregnant, and were either current or recent users of anti-depressants.
Among women who maintained their medication throughout the pregnancy, 26% (21 out of 82) relapsed compared with 68% (44 out of 65) of those who discontinued their medication.
Writing in JAMA, the researchers led by Dr Lee Cohen, said the prevalence of depression in reproductive age women, their level of anti-depressant use and the frequency of unplanned pregnancy, it was important to be able to tell patients about comparative risk of relapse linked to stopping or continuing with their medication.
He added: "With greater awareness and increasing treatment of depression in the community, growing numbers of women may face a clinical decision regarding use of antidepressant medication during pregnancy."
They added: "Such information can also help to refine treatment guidelines for women with a history of depression who are planning to conceive or who experience mood disorders during pregnancy."
Dr O'Keane, a senior lecturer in perinatal psychiatry at the Institute of Psychiatry in London, told the BBC News website: "This paper is very good as it is telling us, in a definitive way, that women with a history of depression should be extremely cautious about discontinuing their medication.
"If they do so, they only have a one in three chance of being well."
She added: "The problem we have as psychiatrists is that women discontinue their medication during pregnancy - for no particularly good reason.
"We then end up having to treat them with higher doses."
Dr O'Keane said: "There was an assumption that pregnancy was protective, but that really isn't the case.
"Pregnancy is another stress, which could worsen their condition."
Dr O'Keane said major depression in pregnancy had been seen to lead to babies being born early and underweight, possible because of the effects of exposure to high levels of stress hormones.