A DRUG commonly used to prevent pregnant women from going into labour too early actually has the opposite effect, a trial has shown.

The antibiotic metronidazole is given to about one in every 200 women to treat a condition called bacterial vaginosis. This is linked to pre-term delivery, so the assumption has been that treating it reduces the risks of having a baby too early. A new trial supported by Tommy’s, the baby charity, suggests that this is mistaken.

The trial identified 900 women over a period of 4½ years who were believed to be at risk of early delivery. At 23 to 24 weeks’ gestation, the women were given a week’s course of either the drug or a placebo. In the placebo group, 39 per cent of the women went on to have pre-term babies. But in the drug group, 62 per cent did, Andrew Shennan and colleagues report in the British Journal of Obstetrics and Gynaecology.

In Britain about one in 20 pregnant women is screened for bacterial vaginosis, and one in 10 of the tests records a positive result. That means that one in 200 pregnant women is BV-positive and at high-risk of having a pre-term baby. Given that there are about 700,000 births each year in Britain, Tommy’s estimates that 1,000 babies may be being born prematurely every year because of the drug.

Metronidazole has been used for years after studies in the 1980s suggested that it was beneficial.

“There is no doubt that bacterial vaginosis is associated with pre-term birth,” Professor Sherman said. “It is not really an infection, but an excess growth of certain bacteria in the vagina that may in fact be normal for some women. Getting rid of this by using metronidazole may not be helpful because it allows other bacteria in.”

It is also possible, the team suggests, that dying bacteria may result in an inflammatory response that increases the risk of premature labour.

Professor Shennan said that the drug also increased the risks of seriously early births, which frequently produced babies that would later suffer severe abnormalities. “Clinicians and high-risk pregnant women should be aware of this research so that we can avoid the escalation of pre-term birth and save more babies’ lives.”

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