Pregnant women who are overweight or obese are at a higher risk of being going into labor ahead of the expected due date, a new study says.

The study, published in the Journal of the American Medical Association (JAMA), analyzed over 1.5 million births in Sweden looked at the weight of the mother and when her baby was born. Researchers said the baby is put at risk as an overweight or obese mother suffers from higher blood pressure, insulin resistance, high cholesterol and increased chance of infection. All these factors directly impact the mother's ability to carry the baby the regular 40 weeks of gestation.

"Considering the high morbidity and mortality among extremely preterm infants, even small absolute differences in risks will have consequences for infant health and survival," the authors wrote. "Even though the obesity epidemic in the U.S. appears to have leveled off, there is a sizable group of women entering pregnancy with very high body mass index (BMI)."

The study, led by Sven Cnattingius, MD, PhD, of the Clinical Epidemiology Unit at Karolinska University Hospital in Sweden, looked at the link between obesity during pregnancy and giving birth early. Obese and severely obese women had a two to three times greater risk of extreme preterm delivery (at 22 to 27 weeks of gestation) as compared with normal-weight women, the study noted.

The researchers analyzed approximately 1.6 million deliveries of single children to women in Sweden between 1992 and 2010. As a woman's weight at her first prenatal visit increased beyond a normal body mass index (BMI) - a measure of weight in relation to height - the researchers found her risk of delivering prematurely increased as well. In their analysis, the researchers took into account the women's age, number of previous children, smoking history, education, height, country of birth and year she gave birth.

The study was titled Maternal Obesity and Risk of Preterm Delivery and was funded by grants from the Karolinska Institutet.