iStock/Thinkstock(NEW YORK) — When pregnant women become ill with infections, many may fear taking antibiotics could endanger the pregnancy. Studies of pregnant women and antibiotics, which are very limited and usually small in size, have shown conflicting evidence about the risks.

To better understand whether antibiotics might present risk, researchers at the University of Montreal published a new study looking at a group of women who had miscarriages and the different types of antibiotics they were prescribed for sickness during their pregnancies.

Antibiotics are an essential mainstay in treating pregnant women, who are often vulnerable to infections due to weakened immune systems. Left untreated, bacterial infections can lead to complications that threaten both the life of the mother and the child.

Researchers found that some common antibiotics increased the risk of a woman miscarrying, however the overall risk was extremely low.

Anick Bérard, Ph.D. expert in fetomaternal and neonatal pathologies at the University of Montreal, co-author of the study published Monday in the Canadian Medical Association Journal, said the risk of having this type of miscarriage, also known as spontaneous abortion, is already low so these findings should not cause undue alarm.

“Taking one of those drugs, the increase in the risk of spontaneous abortion, it’s incremental,” Bérard told ABC News. “It’s a rare event to begin with, so no one should panic.”

The researchers from the University of Montreal looked at data on pregnant women from the Quebec Pregnancy Cohort, between the years of 1998 and 2009. They analyzed nearly 96,000 pregnancies that ended in a miscarriage, as identified in a clinic, comparing more than 8,700 women who had taken antibiotics to more than 87,000 in whom no antibiotic use was reported.

Bérard, along with her co-authors at the University of Montreal, found that certain classes of antibiotics — macrolides, tetracyclines, fluoroquinolones, sulfonamides and metronidazole -— were associated with an increased likelihood of clinically-detected miscarriage before 20 weeks of pregnancy.

Antibiotics routinely used to treat infections ranging from respiratory illnesses to urinary tract infections —- such as Z-Paks (azithromycin, a macrolide), Flagyl (metronidazole), Bactrim (sulfamethoxazole / trimethoprim, sulfonamides) — were all associated with an increased risk for miscarriage.

Only erythromycin, a type of macrolide often used to treat bronchitis, and nitrofurantoin, a type of antibiotic often used to treat urinary tract infections, were not associated with miscarriage.

Bérard said the evidence should actually be reassuring because the study largely confirmed existing guidelines on antibiotic usage for pregnant women. Tetracyclines and fluoroquinolones, for example, are already not recommended during pregnancy, as both have been associated with birth defects.

“Infections should be treated during pregnancy, but it’s a question of what we’re treating with,” Bérard said. “We have to remember that antibiotics are taken for short period of time, for five to ten, max 15 days.”

She said the study can help doctors make informed decisions if they are treating common infections during pregnancy such as urinary tract infections (UTIs).

“The question is, how should they be treated? Our study found that penicillins and cephalosporins are not associated with risk of spontaneous abortion,” Bérard said. “Similarly, we found no risk with erythromycin and nitrofurantoin, these are some of the most-used drugs used to treat UTIs, so our study shows that they are real treatment options.”

The study’s findings may be a start, but they are too preliminary to immediately change how doctors prescribe medication for pregnant women, according to Dr. R. Phillips Heine, a member of the American College of Obstetrics and Gynecology Committee on Obstetric Practice and a Maternal-Fetal specialist at Duke University.

The methods used in the study could have affected the findings, according to Heine, which are a limitation. For example, the doctors may not have been aware of the pregnancies before they prescribed the medication. Additionally, some of the miscarriages may have been misdiagnosed as infections.

“I am very concerned that many of these antibiotic treatment patients could already be having threatened abortions and they were mistaken for infections … like cervicitis, UTI, and vaginitis,” he said.

The study joins a growing body of studies that have offered conflicting information about the risks of taking certain antibiotics during pregnancy.

The difficulty of understanding how drugs affect pregnant women and a developing fetus is bigger issue. Less than 10 percent of medications approved by the U.S. Food and Drug Administration (FDA) between 1980 and 2010 have enough information to determine their risk for birth defects.

“Pregnant women are not included in randomized control trials before a drug becomes available in market,” Bérard said. “But, infections are very prevalent during pregnancy, and antibiotics will be prescribed. Studies like ours give treatment options.”

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