NEW YORK (CNNMoney.com) - Paxil, a blockbuster antidepressant from GlaxoSmithKline, increases the risk of babies' heart defects in pregnant mothers, the Food and Drug Administration said Thursday.

At the request of the FDA, GlaxoSmithKline has strengthened the warning label on Paxil to show that studies in pregnant women have demonstrated a risk to the fetus, according to the company and the FDA.

"We obviously have worked with the FDA and we're moving forward to make this information available to health care professionals and patients so that informed treatment decisions can be made," said GlaxoSmithKline spokeswoman Gaile Renegar.

The FDA said a study showed that pregnant women taking Paxil during the first trimester had a 2 percent chance of developing a heart defect, compared to 1 percent for the general population. A second study showed a 1.5 percent chance of developing defects with Paxil, compared to a 1 percent chance in expectant mothers taking other types of antidepressants, said the FDA.

Most of the defects were holes in the chambers of the heart, the FDA said.

GlaxoSmithKline changed its label in September, when it first referenced the two studies as "preliminary." Renegar said the label now carries "more expansive descriptions" of the two studies and their findings, in addition to the ramped-up warning.

GlaxoSmithKline's (up $0.63 to $50.80, Research) stock price made moderate gains early Thursday.

Based in London, GlaxoSmithKline sales totaled $39 billion in 2004, with more than $1 billion from Paxil.