Washington: Women who are overweight or obese are more likely to have preterm delivery, according to a new Swedish study.

Those with the highest Body Mass Index (BMI) also had the highest statistical risk of giving preterm birth and especially extremely preterm birth.

"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," said Dr. Sven Cnattingius, Professor at Karolinska Institute in Sweden, who led the study.

In the study, Dr. Cnattingius and colleagues at Karolinska Institute and the University of Michigan, U.S., used information from 1.5 million singleton deliveries included in the population-based Swedish Medical Birth Register from 1992 through 2010.

Compared to women of normal weight, overweight women had a 25 percent increased risk of extremely preterm delivery. Women with mild obesity had a 60 percent increased risk of giving birth extremely preterm. For women with severe obesity (BMI 35-39.9) or extreme obesity (BMI 40 or more) the corresponding risk was doubled and tripled, respectively.

Risks of very and moderately preterm deliveries also increased with BMI. Overweight and obesity also increase the risk of maternal pregnancy complications, including preeclampsia, gestational diabetes, and Cesarean delivery, said Dr. Cnattingius.

Infection and inflammation are considered main risk factors for spontaneous extremely preterm delivery with a spontaneous onset, and maternal obesity is associated with increased production of inflammatory proteins. The researchers hypothesize that the increased inflammatory state in obese women may make them more susceptible to infections, which may increase their risk of spontaneous extremely preterm delivery.