Up to 1,000 babies are born prematurely each year because of a drug that is prescribed to their mothers during pregnancy to prevent the early onset of labour, researchers say.
Metronidazole, an antibiotic, has been widely prescribed since the 1980s to treat pregnant women who have a bacterial vaginal infection. There is a known link between such infections and early labour.
But researchers from the babies' charity Tommy's who looked at 900 women at risk of giving birth prematurely found that those in the highest risk group who took the drug were twice as likely to experience early labour as those who went untreated.
About one in 200 pregnant women is prescribed the drug, in most cases for bacterial vaginosis (BV).
The researchers said their study indicated that about 1,000 babies were being born early because of the drug, and they advised that doctors stop prescribing it to give babies already at risk a greater chance of survival.
The 900 women were those at risk of going into labour between the 23rd and 24th week. Of those, the team selected the 100 most at risk, who were already showing very early indications of labour. Half were given metronidazole and half a placebo. Only 39% of women in the placebo group had their baby pre-term, compared with 62% of those on the antibiotic.
Andrew Shennan, author of the research and obstetrics professor at St Thomas' hospital, London, said: "Metronidazole shouldn't be given to women routinely to prevent pre-term labour. There are times when it may be important for treatment of some illnesses but certainly not as a prophylactic for labour. There are other alternative treatments."
Other specialists suggested that the higher incidence of pre-term labour associated with metronidazole could be linked to the disruption to the body's natural bacteria levels that the drug causes.
Maggie Blott, a consultant obstetrician at the Royal Victoria infirmary in Newcastle, said: "Metronidazole changes the bacteria in the vagina and by doing that you increase the risk of other types of bacterial infections."
Tommy's said that about 5% of the antenatal population was screened for BV, with one in 10 tests proving positive. This meant that one in 200 pregnant women were at high risk of delivering prematurely and of being BV positive. Most of these women will be prescribed metronidazole and 30% will go on to have a pre-term baby.
Richard Warren, honorary secretary of the Royal College of Obstetricians and Gynaecologists, said: "This raises serious questions about metronidazole ... It does not mean that all antibiotics during pregnancy are dangerous but it does ring alarm bells about whether it should be prescribed specifically to prevent pre-term labour."
