LONDON (Reuters) - A once-used practice of giving antibiotics to some women at risk of premature birth may have increased the odds their children will develop cerebral palsy and other problems, British researchers said on Thursday.

The study published in the journal Lancet looked at women at risk of premature labor who had no signs of infection. Doctors now recommend antibiotics only for women whose waters have broken prematurely or have an obvious infection.

The findings reaffirm that doctors should not use antibiotics for premature labor when the mother's water is intact and if there is no infection, said Sara Kenyon, a researcher at the University of Leicester who led the study.

"We don't think it is the antibiotics themselves but rather the situation the antibiotics are given in," she said in a telephone interview. "These findings mean doctors do not need to give antibiotics if a woman's water hasn't broken unless she has an infection."

Smoking, alcohol use and weight problems can increase the chances of premature labor. While it was known children born prematurely are more prone to functional problems later in life, the link to cerebral palsy was unexpected, Kenyon said.

Kenyon's team followed up 9,000 children from the original trial at age 7. The researchers used a health questionnaire and national school results to gauge the children's health.

Children whose mothers were given the antibiotic erythromycin had an 18 percent higher risk of mainly mild functional problems that also included struggles with day-to-day problem solving compared to those whose mothers did not receive the drug.

The other antibiotic, co-amoxiclav, did not appear to raise such risk.

"The risk of cerebral palsy was increased by either anti-biotic, although the overall risk of this condition was low," the researchers wrote in the report.

But for women whose water did not break and who got both antibiotics together, the chance their children would develop cerebral palsy nearly tripled.

The researchers said the overall risk was low and that they do not know why the combination of antibiotics and an intact membrane in women whose water did not burst appeared to affect some children.

(Reporting by Michael Kahn, Editing by Maggie Fox )