Some of the antibiotics used to treat urinary tract infections during pregnancy may increase the risk of several birth defects if a woman uses them early in pregnancy, a new study in the Archives of Pediatrics & Adolescent Medicine shows.

Researchers found an increased risk for two classes of antibiotics: sulfonamide (example: Bactrim) and nitrofurantoins (example: Macrobid). But the antibiotics pregnant women are most likely to be prescribed, the penicillins and erythromycins, appeared to be safe.

Most of the commonly used antibiotics do not seem to be associated with most of the birth defects we studied, study co-author Dr. Krista S. Crider, of the Centers for Disease Control and Prevention in Atlanta, told Reuters Health. This should be helpful for pregnant women and their health care provider as they're trying to make decisions on treatment of infections during pregnancy.

Bacterial infections can be extremely dangerous for a pregnant woman and her fetus, and should not go untreated, Crider and her team note in their report. But to date, they add, there have been no large-scale studies of whether or not antibiotics are safe in pregnancy.

To investigate, they looked at use of six different classes of antibiotics in more than 13,000 women whose pregnancies were affected by at least one of 30 major birth defects, and close to another 5,000 who had babies free from birth defects.

About 30 percent of the women in each group used an antibiotic at some point in her pregnancy, or during the previous three months.

But the researchers were interested in the month before pregnancy and the first trimester, a critical time in fetal development. Around 14 percent of the birth defect group had used an antibiotic during this period, as had 13 percent of the control group.

Sulfonamides were linked to six different birth defects, while nitrofurantoins were linked to four. Other classes of antibiotics were associated with one or two birth defects.

Crider said these findings should not be cause for concern among women who may need to take these drugs. For example, there were around 1,000 women who took penicillin, and just four had pregnancies affected by the birth defect in question. Those are some pretty low numbers, the researcher said.

It's important to realize that in every pregnancy, regardless of any medication use, there's a 3 percent chance of a birth defect, Crider added. Many of the defects that we looked at in this study were very rare.

For example, she noted, a severe heart abnormality known as hypoplastic left heart syndrome, which was three times as common among women who took sulfonamides and four times more frequent among those who used nitrofurantoins, occurs in one in every 42,000 births. If risk is tripled, Crider pointed out, that still means only one case in every 14,000 births.

According to Crider, more research is needed before specific recommendations can be made on which antibiotics are safe-or not safe-to take in pregnancy. She said several years of data now coming in from the National Birth Defects Prevention Study may help to answer this question.

SOURCE: Archives of Pediatrics & Adolescent Medicine, November 2009.