An antibiotic prescribed to pregnant women to prevent preterm labor might actually cause it, according to a study published in the Jan. 1 edition of BJOG: An International Journal of Obstetrics and Gynaecology, Reuters reports. Metronidazole -- a drug that has been commonly prescribed since the 1980s to prevent pregnant women with bacterial vaginosis from delivering their infants early -- might double the risk of early delivery, the study finds (Reuters, 1/12). Researchers from Tommy's, a United Kingdom-based not-for-profit group that funds research to prevent stillbirths and miscarriages, studied 900 pregnant women "at risk" of pre-term labor, according to London's Guardian. Andrew Shennan, a professor at St. Thomas' Hospital in London, and colleagues identified the 100 women at highest risk, and randomly divided the group so that half took Metronidazole and half received a placebo (Curtis, Guardian, 1/13). The study finds that 39% of the women who took the placebo delivered preterm, compared with 62% of those who took metronidazole (Reuters, 1/12). "Instead of one in three women at high risk [of] giving birth prematurely, the proportion was two-thirds," Shennan said (Daily Mail, 1/13). Shennan added that metronidazole should not be recommended to prevent early labor (Reuters, 1/12). According to Tommy's, about 5% of pregnant women in the U.K. are tested for BV, with one in every 10 testing positive. Most women who test positive are prescribed metronidazole and 30% deliver prematurely, the Guardian reports. While the study raises questions about the use of metronidazole for pregnant women, "[i]t does not mean that all antibiotics during pregnancy are dangerous," Richard Warren, honorary secretary of the Royal College of Obstetricians and Gynaecologists said (Guardian, 1/13). Jim Kennedy, spokesperson for the Royal College of General Practitioners, said that any drug carries potential risks, as well as benefits, adding that the drug "also reduces the risk of infection, so ... you have to consider whether the risk of an earlier birth is worse than protecting" the infant from infection (BBC News, 1/13).