Overweight and obese women are more likely to deliver their babies preterm, and the higher the mother's body mass index, the greater the risk of extremely preterm delivery, suggests a new study in the Journal of the American Medical Association. Researchers evaluated data from nearly 1.6 million births in Sweden from 1992 to 2010. Among normal-weight women (with a BMI greater than 18.5 and less than 25), the rate of extremely preterm delivery -- less than 28 weeks of gestation -- was 0.17%. Compared with normal-weight women, women with a BMI of 25 to less than 30 were 26% more likely to deliver extremely preterm babies, while women with a BMI of 35 to less than 40 were twice as likely, and those with a BMI of 40 or greater were three times as likely. In the U.S., where the majority of women are overweight or obese in early pregnancy, extremely preterm birth is a leading cause of infant death and long-term disability. Read the abstract.