A drug prescribed to pregnant women at risk of premature birth may actually do more harm than good, a study suggests.
One in 200 pregnant women are given the drug
In a study of 900 women, baby charity Tommy's found antibiotic metronidazole doubled the risk of pre-term delivery.
The report in the International Journal of Obstetrics and Gynaecology said this meant 1,000 babies were born pre-term yearly in the UK because of the drug.
But doctors said the drug could cut the risk of infection in the baby and that had to be taken into account.
Researchers studied 900 women who were recruited between 23 and 24 weeks of pregnancy and at known risk of pre-term birth.
They were split into two groups, with half receiving a week's course of metronidazole and the other given dummy drugs.
The results suggested that not only was metronidazole ineffective in preventing pre-term births, it may also be detrimental.
Only 39% of women in the placebo group had their baby pre-term, compared with 62% of those who took the antibiotic.
The researchers said it was thought that metronidazole may be leading to a greater incidence of pre-term births in high-risk women.
The drug is generally prescribed for the condition bacterial vaginosis (BV), which is an infection linked with premature deliveries.
Tommy's said that about 5% of the antenatal population were screened for BV, with one in 10 tests coming out positive.
This meant that one in 200 pregnant women were at high risk of delivering prematurely and given there were about 700,000 births a year in the UK, Tommy's estimated about 1,000 babies may be being born prematurely because of the drug.
Report author Andrew Shennan, professor of obstetrics for Tommy's, said: "Clinicians and high-risk pregnant women should be aware of this research outcome so that we can avoid the escalation of pre-term birth and, in turn, save more babies' lives."
But Dr Jim Kennedy, prescribing spokesman for the Royal College of GPs, said any drug carried risks and potential benefits had to be weighed against disadvantages.
"The drug also reduces the risk of infection, so if a woman is at 35 weeks or so you have to consider whether the risk of an earlier birth is worse than protecting the baby against infection."
A spokesman from the Medicines and Healthcare products Regulatory Agency (MHRA) said the product information currently advised doctors to exercise caution or avoid using metronidazole-containing products in pregnant women.
"Some doctors, however, may decide to use metronidazole in pregnancy if they consider it to be in the best interests of their patient.
"The MHRA will carefully review this new study and consider whether further advice is necessary."
"Pregnant women who have been prescribed metronidazole should not
stop their treatment without seeking medical advice."