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Penicillin, which is the most commonly used antibiotic during pregnancy, as well as erythromycin, cephalosporins and quinolones, other widely prescribed antibiotics, were not associated with increased risk for about 30 different birth defects.
However, the study found that two types of antibiotics were linked with a higher risk for several birth defects: nitrofurantoins and sulfonamides, sometimes called "sulfa drugs," which are prescribed for urinary tract and other infections.
Women whose children had anencephaly, a fatal malformation of the skull and brain, were three times more likely to have taken sulfonamides, the study found. Sulfonamides were also tied to an increased risk for such heart defects as hypoplastic left heart syndrome and coarctation of the aorta, choanal atresia (a blockage of the nasal passage), transverse limb deficiency and diaphragmatic hernia, an abnormal opening in the diaphragm that results in severe breathing difficulties.
Nitrofurantoins were also associated with multiple birth defects, including anophthalmia and microphthalmos (eye defects) and several congenital heart defects. Mothers whose children were born with a cleft lip or cleft palate were twice as likely to have taken nitrofurantoins, the study found.
But pregnant women should not be overly worried if they need an antibiotic to treat an infection during pregnancy, stressed the study's lead author, Krista Crider, a geneticist with the National Center on Birth Defects and Developmental Disabilities, part of the U.S. Centers for Disease Control and Prevention.
"The most important message is that most commonly used antibiotics do not seem to be associated with the birth defects we studied," Crider said.
The findings are published in the November issue of Archives of Pediatrics & Adolescent Medicine.
Crider and her colleagues analyzed data on more than 13,000 women whose babies had one of more than 30 birth defects, including cleft palate, heart or limb defects and anencephaly. They compared the women's rates of antibiotic usage, from the month leading up to pregnancy through the end of the first trimester, with that of almost 5,000 women whose children did not have a birth defect. The data was culled from the National Birth Defects Prevention Study, which began in 1997 and includes about 30,000 women from 10 states.
Information on the impact of many prescription drugs on developing fetuses is sorely lacking, Crider pointed out. Much of that stems from the fact that ethical considerations preclude conducting drugs trials in pregnant women, she said.
Though many antibiotics have been used safely for decades, resistant strains of bacteria are forcing doctors to use a wider array of antibiotics. For some, little data exist.
The researchers found that about 30% of women took an antibiotic between the three months prior to conception and the end of the pregnancy.
Even antibiotics that generally were safe were found to be associated with a few specific birth defects. Women whose babies were born with a certain type of limb malformation were three times more likely to have taken penicillin. Erythromycin, cephalosporins and quinolones were also associated with an increased risk for one or two specific birth defects.
However, the researchers said they did not know if the birth defects were caused by the antibiotics or the underlying infection.
One expert said women need to remember the good antibiotics can do mom and baby, as well. Though many pregnant women want to avoid taking any drugs during pregnancy, infections pose a risk to mother and baby and often need to be treated, said Dr. Jennifer Wu, an obstetrician-gynecologist at Lenox Hill Hospital in New York City.
"Untreated infections during pregnancy can lead to severe consequences, such as maternal sepsis [blood infection] and preterm labor," Wu said. "Yet many patients are afraid to take medications such as antibiotics during pregnancy."
The study "supports the evidence that antibiotics are safe for pregnant women," she said. "It is reassuring for doctors and patients to have more data on necessary drugs for pregnancy."
Crider also stressed that the chances of having a baby with a birth defect remain small, even if an antibiotic has been linked to an elevated risk. For example, the risk of having a child with hypoplastic left heart syndrome is about one in 4,200. Sulfonamides were associated with a three-fold increase, making the likelihood about one in 1,400, she said.
Brand names for nitrofurantoins include Furadantin, Macrobid and Macrodantin. Bactrim and Septra are among the brand names of sulfonamides.
Given the data, Crider said, women should be cautious about taking either of those types of drugs during pregnancy and should discuss other options with their physicians,.
According to the study, the overall risk for having a child with a birth defect is about three percent.
The study did not look at chromosomal defects, including Down syndrome.
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SOURCES: Krista Crider, Ph.D., geneticist, National Center on Birth Defects and Developmental Disabilities, U.S. Centers for Disease Control and Prevention, Atlanta; Jennifer Wu, M.D., obstetrician-gynecologist, Lenox Hill Hospital, New York City; November 2009, Archives of Pediatrics & Adolescent Medicine