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

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."

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. Continued...