updated 11/2/2009 4:06:29 PM ET 2009-11-02T21:06:29

Researchers studying antibiotics in pregnancy have found a surprising link between common drugs used to treat urinary infections with birth defects. Reassuringly, the most-used antibiotics in early pregnancy — penicillins — appear to be the safest.

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Bacterial infections themselves can cause problems for the fetus if left unchecked, experts said, so pregnant women shouldn't avoid antibiotics entirely. Instead, women should discuss antibiotics choices with their doctors.

The new study is the first large analysis of antibiotic use in pregnancy. It found that mothers of babies with birth defects were more likely than mothers with healthy babies to report taking two types of antibiotics during pregnancy: sulfa drugs (brand names include Thiosulfil Forte and Bactrim) and urinary germicides called nitrofurantoins (brand names include Furadantin and Macrobid).

It was the first time an association had been seen between urinary tract treatments and birth defects, said lead author Krista Crider, a geneticist with the Centers for Disease Control and Prevention, which funded the research. "Additional studies are going to need to be done to confirm these findings."

Before rigorous safety testing
Used for many decades, the antibiotics in question predate the Food and Drug Administration and its requirements for rigorous safety testing. The FDA now grades all drugs for safety to the fetus based on available research, but rigorous studies are so lacking in many cases, that no antibiotics get the highest grade of "A."

Sulfa drugs are the oldest antibiotics and some animal studies have found harm during pregnancy. Nitrofurantoins previously have been viewed by doctors as safe to treat urinary tract infections during pregnancy.

The study, appearing in November's Archives of Pediatrics and Adolescent Medicine, may cause doctors to change the drugs they choose to treat pregnant women with infections. The findings were released Monday.

Dr. Susan Mehnert-Kay, a family practice doctor in Tulsa, Okla., who has written about diagnosing and managing urinary tract infections, said the research is "very interesting" and would cause her to reconsider antibiotic choices in early pregnancy.

The study is important because it looked at drugs that have been used for decades without large studies of their safety in pregnant women, said Dr. Michael Katz of the March of Dimes.

"Some physicians are not as attuned to this as they ought to be, so patients have the right to ask questions," Katz said.

The researchers analyzed data from more than 13,000 mothers whose infants had birth defects and nearly 5,000 women who lived in the same regions with healthy babies.

The women were interviewed by phone from six weeks to two years after their pregnancies. Those who remembered taking antibiotics during the month before conception through the first three months of pregnancy were identified as exposed to antibiotics.

The women's memories could have been faulty, a substantial weakness of the study, which the authors acknowledged. About one-third of the women who took antibiotics couldn't remember the specific type of drug they took.

It's also unclear whether the birth defects were caused by the drugs or by the underlying infections being treated, Crider said.

Birth defects linked to sulfa drugs included rare brain and heart problems, and shortened limbs. Those linked to nitrofurantoins included heart problems and cleft palate. The drugs seemed to double or triple the risk, depending on the defect.

"These defects are rare. Even with a threefold increase in risk, the risk for the individual is still quite low," Crider said.

Katz of the March of Dimes said anencephaly, a fatal brain problem linked to sulfas, affects about 1 in 10,000 births in the United States. Cleft palate occurs about 20 per 10,000 births.

Crider said the findings give doctors another opportunity to caution against overuse of antibiotics. Viral illnesses like colds and flus shouldn't be treated with antibiotics, she said.

Women in 10 states, including California, Texas and New York, were interviewed as part of the National Birth Defects Prevention Study.

The FDA recommends that pregnant women discuss medications with their doctors, said FDA spokeswoman Sandy Walsh. The agency has proposed changes to prescription drug labeling that would require more complete information for women of childbearing age, pregnant women and those who breastfeed, Walsh said.

© 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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