Obese or overweight mothers are more likely to give birth prematurely, a new study has warned.
Swedish researchers found the risk of preterm delivery increases with maternal overweight and obesity.
Women with the highest Body Mass Index (BMI) also had the highest statistical risk of giving preterm birth - and especially extremely preterm birth, the study found.
"For the individual woman who is overweight or obese, the risk of an extremely preterm delivery is still small," said Dr Sven Cnattingius, Professor at Karolinska Institutet in Sweden, who led the study.
"However, these findings 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," Cnattingius said.
Cnattingius and colleagues at Karolinska Institutet and the University of Michigan, US, used information from 1.5 million singleton deliveries included in the population-based Swedish Medical Birth Register from 1992 through 2010.
Information about maternal weight at first visit to prenatal care and height was used to calculate women's BMI, defined as weight in kilogrammes divided by height in square metres.
A BMI between 18.5 and 24.9 was assessed as normal, 25 to 29.9 as overweight, and a BMI of 30 or more as obesity. Among young Swedish women of normal length (167 cm), a weight from 70 to 83 kilogrammes is considered overweight and more than 83 kilogrammes is considered obesity.
Compared to women of normal weight, overweight women had a 25 per cent increased risk of extremely preterm delivery.
Women with mild obesity had a 60 per cent 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.
"34 per cent of all pregnant women in Sweden are either overweight or obese, and this impacts the number of preterm born infants," said Cnattingius.
"Overweight and obesity also increase the risk of maternal pregnancy complications, including preeclampsia, gestational diabetes, and Cesarean delivery," Cnattingius said.
The researchers hypothesise 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.
The study was published in Journal of the American Medical Association (JAMA).
