Giving antibiotics to pregnant women in the hopes of delaying labour may increase the child's risk for cerebral palsy, a new study suggests.

Doctors used to give antibiotics to women at risk for premature labour who had no signs of infection. The drugs are now only recommended when a woman's water breaks prematurely or if she has an obvious infection.

In Thursday's online issue of the medical journal the Lancet, British researchers looked at whether prescribing two antibiotics could prevent underlying infection to delay premature birth in 4,221 pregnant women.

Children born to the women were followed up for seven years, using health questionnaires and national school results.

The benefits of antibiotics were deemed to outweigh the risk of complications when a woman showed signs of infection or if her water had broken.

But if that wasn't the case, the children of women who had been given antibiotics were at increased risk of poor eyesight, mild physical and cognitive impairment that interfere with day-to-day problem solving and cerebral palsy, Sara Kenyon of the University of Leicester and her colleagues found.

Overall risk low

The researchers found that 4.4 per cent of children whose mothers had taken both the antibiotics erythromycin and co-amoxiclav had cerebral palsy, compared with 1.6 per cent of those born to mothers who took a placebo, nearly triple the risk.

Among children whose mothers took the antibiotic erythromycin, 42.3 per cent had mild functional problems, compared with 38.3 per cent for those born to women who did not receive the antibiotic.

"The risk of cerebral palsy was increased by either antibiotic, although the overall risk of this condition was low," the researchers wrote.

The risk of cerebral palsy, which affects movement and muscle co-ordination, is higher among babies born early.

About two to three children in 1,000 are affected by cerebral palsy, according to the U.S. Centers for Disease Control. Among children in the study, the risk was about two in 100, which rose to three to four in 100 for those whose mothers took one or both antibiotics, the researchers said.

"These findings do not mean that antibiotics are unsafe for use in pregnancy," Britain's Royal College of Obstetricians and Gynaecologists said. "Pregnant women showing signs of infections should be treated promptly with antibiotics."