Doctors had no reason to believe ACE inhibitors caused defects early in pregnancy because they have had little research to go on. It's considered too risky to study medications in pregnant women.

The drugs, called ACE inhibitors, are effective and well-tolerated by most patients. But they already carry a "black box" advisory that warns women to stop taking them when they learn they're pregnant.

If the drugs are taken during the second or third trimesters, they can cause neonatal kidney failure, growth retardation and fetal death, studies indicate.

Lack of Research 'Shocking'

Doctors had no reason to believe ACE inhibitors caused defects early in pregnancy because they have had little research to go on. It's considered too risky to study medications in pregnant women.

"This is not the last word on the subject, but it is shocking to realize that it is almost the first," Jan Friedman of the University of British Columbia, Vancouver, wrote in an editorial with the study in Thursday's New England Journal of Medicine.

"The research involved 29,507 infants who were born between 1985 and 2000 and enrolled in Tennessee's Medicaid program. Mothers of 209 infants took ACE inhibitors during the first trimester; 202 took other blood pressure drugs, and 29,096 did not take any blood pressure drugs.

Rising Obesity a Worry

Lead investigator William Cooper of Vanderbilt University and his team identified birth defects in birth, death and fetal-death records and used medical charts to confirm the reports.

The study found that 18 infants whose mothers took ACE inhibitors had major birth defects, most of them involving the heart or central nervous system, for a rate that was 2.7 times higher than women who took other high blood pressure drugs or no blood pressure medications.

Christopher Cannon of Brigham and Women's Hospital says, "This may become more of a problem as cardiovascular disease moves into younger and younger patients," partly because of rising obesity.

Taken together, all of the drugs in this class accounted for $5 billion in sales last year, up 13 percent from 2004, according to market research firm IMS Health.