First-time mothers who are obese are almost twice as likely to have premature and low birthweight babies and have a higher risk of pre-eclampsia, research suggests.

Those with a body mass index (BMI) of more than 30 carry a particularly high risk, according to a study of 385 obese British and Dutch women in their first pregnancy.

Of the babies born to these women, 18.8 per cent were of lower than normal birthweight compared with a rate of around 10 per cent for the general population. A total of 13.4 per cent were higher than normal birthweight; the average is 10 per cent.

And 11.7 per cent of the women developed pre-eclampsia, compared with 2 per cent of the average population and 6 per cent of obese women with one or more previous pregnancies. The rate of pre-eclampsia increased as BMI rose, according to the study published in the American Journal of Obstetrics and Gynaecology.

The premature birth rate in the group was 11.9 per cent; the national average is 7 per cent.

The Caesarean section rate of 39 per cent was the highest ever reported in the world, the authors said. The higher their BMI, the more mothers were likely to need to stay in hospital.

Lucilla Poston, who led the research, funded by the baby charity Tommy's, the Wellcome Trust and the Biomedical Research Centre, at Guy's & St Thomas' NHS Foundation, said the findings caused concern.

“We must now start to consider first-time pregnancy as an additional problem in obese pregnant women,” she said. “The large proportion of small babies was particularly unexpected, as obesity is more often associated with the birth of overweight babies.”

The rate of premature births rose from 7 per cent in 2006, almost 48,000 babies, to 8.6 per cent in 2007.

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