The largest study ever done on the causes of birth defects in the United States has found that women who took opioid painkillers during early pregnancy or just before becoming pregnant had nearly double the risk of bearing children suffering from one of several severe birth defects.

The actual risk increase to an individual woman is small. But because the birth defects are so serious, all women should talk with their doctor before taking opioids during pregnancy.

Opioid painkillers include all preparations that contain codeine, oxycodone or hydrocodone.

The CDC, which conducted the study, offers two interpretations of these results. It stresses that the actual risk increase to an individual woman is small. But because the birth defects are so serious, all women should talk with their doctor before taking opioids during pregnancy.

The study found risks for five serious birth defects were roughly doubled for women who took prescription opioids from one month before pregnancy to the end of the first trimester. The birth defects included congenital heart defects, spina bifida and hydrocephaly.

Thomas R. Frieden, director of the CDC, takes the results seriously. According to Frieden, "Women who are pregnant, or thinking about becoming pregnant, should know there are risks associated with using prescription painkillers. They should only take medications that are essential, in consultation with their health care provider."

The National Birth Defects Prevention Study is an ongoing population-based study, with ten states currently participating: Arkansas, California, Georgia, Iowa, Massachusetts, New Jersey, New York, North Carolina, Texas and Utah. For the opioid portion of the study, researchers looked at data from 17,449 women who bore children with birth defects and 6,701 women who gave birth to a healthy baby.

A little over 2% of these women reported opioid use during pregnancy. Those who did were about twice as likely to have children with one of five serious birth defects.

Cheryl Broussard, the study's lead author, echoes Frieden's recommendations. Broussard also cautions that it's important for any woman who is pregnant or planning to become pregnant to talk to her doctor before taking any medication — prescription, over-the-counter or herbal.

It has not yet been determined if most medications are safe to take during pregnancy. Most clinical trials exclude pregnant women from taking part in them.

The CDC issued its findings in a press release on March 2. Dr. Broussard's article was published online by the American Journal of Obstetrics and Gynecology on the same date.

Cheryl S. Broussard, PhD, is an epidemiologist with the CDC's National Center on Birth Defects and Developmental Disabilities (NCBDDD).