Scientists have warned that overweight mothers are more like to have premature babies.
Women who were overweight - meaning a Body Mass Index (BMI) of between 25 and 30 - were 25 per cent more likely to have an extremely premature baby which is more likely to struggle for survival, compared with mothers of healthy weight.
With a BMI of between 30 and 35 (obese) the risk was 60 per cent greater, rising to 100 per cent greater for those with a BMI of 40 or more.
Experts said the 18 year study of Swedish mothers, published in the Journal of the American Medical Association, sounded a warning to women in the UK, where three in five adults are now overweight or obese.
Britain has the highest number of premature babies born in Europe, with 40,000 premature births each year, figures show.
The study found that much of the risk was caused because obesity increases the risk of severe complications in pregnancy, such as pre-eclampsia, which mean babies have to be delivered earlier.