Severely obese women who become pregnant are more likely to need to deliver their infants prematurely because of complications such as high blood pressure, and their babies face a greater risk of death or long-term disability, a new study shows.

While the risk of elective preterm delivery is greater in obese women who are pregnant for the first time, excess weight has no apparent effect on preterm delivery risk among women who have already had one or more children, Dr. Gordon C. S. Smith of Cambridge University in the UK and colleagues found.

Obesity is defined as a body mass index (BMI) between 30 and 34.9, and morbid or severe obesity is a BMI of 35 or greater. In elective preterm delivery, doctors induce labor or deliver a baby by C-section before 37 weeks of pregnancy, often because the woman has a condition called preeclampsia that puts her in danger of potentially fatal seizures during labor.

Past research on obesity and pregnancy has yielded mixed results, with some studies finding that being heavy reduced a woman's risk of delivering early, Smith and his team note in the American Journal of Public Health.

To better understand the relationship, they looked at BMI and pregnancy outcomes for 187,290 Scottish women.

Regardless of their weight, the researchers found, women pregnant for the first time had a greater risk of an adverse pregnancy outcome. As BMI rose, the risk of spontaneous preterm delivery dropped, especially among women who had already had a baby. But among women pregnant for the first time, the risk of elective preterm delivery rose with BMI.

Morbidly obese women pregnant for the first time were at greater risk of having an elective or a spontaneous preterm delivery, having an infant die before one year of age, and having an extremely low birth weight infant, who has a greater risk of long-term complications such as learning problems.

In contrast, morbidly obese women who had already been pregnant at least once faced no increased risk of any of these negative outcomes.

"These data indicate that morbidly obese women who are planning to conceive should be encouraged to lose weight before their first birth," Smith and his colleagues conclude.