Women's Health: Antibiotics can raise or lower premature birth risk

Certain antibiotics may help pregnant women who are at high risk for delivering their baby too early, while others may actually be harmful, a Canadian study has shown. Up to half of preterm deliveries may be related to infections such as bacterial vaginosis, an overgrowth of harmful bacteria in the vagina. But previous studies have yielded conflicting results about the effects of antibiotic treatments on preterm birth rates, according to Dr. Emmanuel Bujold, assistant professor of obstetrics and gynecology at Laval University in Quebec City. Bujold and his colleagues attempted to clarify the situation by reviewing 16 antibiotic studies involving more than 8,000 women. They found that antibiotics known as macrolides (such as erythromycin) and clindamycin are associated with a decreased risk of preterm birth compared with inactive placebo pills, while metronidazole (sold as Flagyl and other names) is associated with an increased risk of preterm delivery. All of the studies involved pregnant women in their second trimester who were at high risk for delivery before 37 weeks. Risk factors included infections, prior preterm delivery and low maternal weight prior to conception. Bujold says all the antibiotics had been reported as potentially useful to prevent preterm birth, but some studies showed a clear benefit, while others showed no benefit. He suggests the confusion may have arisen because the antibiotics work in different ways. Metronidazole kills bacteria directly, which may lead to the release of inflammatory substances that could increase preterm birth risk. In contrast, clindamycin and macrolides stop the infectious process without killing the bacteria. Clindamycin and macrolides are also more effective than metronidazole against the types of bacteria most frequently linked with preterm birth. Bacterial vaginosis is usually treated with either clindamycin or metronidazole, but Bujold recommends discontinuing the use of metronidazole in pregnant women with this condition. "Bacterial vaginosis is not a rare problem," he says. "Those women are known to be at high risk for preterm birth. And if we increase that risk even more by using the wrong antibiotics, it can be a big problem."