Pregnant women who are overweight or obese are at an increased risk of delivering their babies too early, a study has shown.

US and Swedish scientists used information from 1.5 million births that occurred between 1992 and 2010. The women's body mass index (BMI) was determined at their first prenatal appointment. A BMI of 18.5 to 24.9 is classed as normal, 25 to 29.9 is classed as overweight and 30 or more is considered obese.

The study found that compared to normal weight women, those who were overweight had a 25% increased risk of giving birth extremely prematurely - that is more than 12 weeks before their due date.

Those with mild obesity (BMI of 30-34.9) had a 60% increased risk.

The bigger a woman was, the greater the risk. Those with extreme obesity (BMI of 40 or more) had triple the risk of giving birth extremely prematurely.

Overweight and obese women were also more likely to give birth very prematurely (eight to 12 weeks early) and moderately prematurely (four to eight weeks early).

In many cases, the babies had to be delivered early due to obesity-related pregnancy complications, such as pre-eclampsia (dangerously high blood pressure).

In relation to spontaneous labour, most of these cases related to extreme prematurity. According to the scientists, maternal obesity is linked to an increased production of inflammatory proteins and this increased inflammation may make obese women more susceptible to infections. This in turn may increase the risk of spontaneous extremely premature deliveries.

"For the individual woman who is overweight or obese, the risk of an extremely preterm delivery is still small. However, these finding are important from a population perspective. Preterm infants and, above all, extremely preterm infants account for a substantial fraction of infant mortality and morbidity in high income countries," explained lead scientist, Dr Sven Cnattingius, of the Karolinska Institutet in Sweden.

Details of these findings are published in the Journal of the American Medical Association.

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