The antidepressant Paxil may increase the risk of birth defects when women take it during the first three months of pregnancy, the U.S. Food and Drug Administration warned Thursday.

"FDA is advising health care professionals not to prescribe Paxil in women who are in the first three months of pregnancy or are planning pregnancy, unless other treatment options are not appropriate," a statement from the agency said.

Health Canada issued a similar warning in October and plans to issue a new advisory by the end of December.

The FDA strengthened its warning based on the early results of two studies.

The rate of heart defects in babies was 1.5 to two times greater among pregnant women taking Paxil during the first trimester compared to women taking other antidepressants or the general population.

Most of the problems were ventricular septal defects, or holes in the muscular wall between the heart's left and right lower chambers.

In general, the severity of the defects range from minor ones that may correct themselves to those that cause serious symptoms and need to be repaired surgically, the FDA said.

Women currently taking Paxil should not discontinue treatment without talking to a doctor first, Health Canada and the FDA said.

Paxil and its generic version, paroxetine, "should usually not be taken during pregnancy," the FDA added.

But, in some patients, the benefits of continuing Paxil to relieve depression may be greater than the potential risk to the fetus, the regulator said.

Paxil's manufacturer, GlaxoSmith Kline, and the FDA said they await final results of the studies to better understand the birth defect risk.