The Food and Drug Administration (FDA) on Tuesday posted a warning on its Web site that the popular antidepressant Paxil may increase the risk of birth defects if pregnant women take it during the first trimester. The drug's manufacturer, GlaxoSmithKline sent the agency a letter citing evidence from a new study and indicating it would change the label information to reflect the increased risk possibility. The study involved 3,581 pregnant women and found a 4 percent incidence of birth defects in those taking Paxil during the first trimester, versus a 2 percent incidence in those taking other antidepressants. The most common malformations were ventricular septal defects: holes or other flaws in the wall between two of the heart's chambers.