FRIDAY, Sept. 20 (HealthDay News) -- Pregnant women with a body-mass index (BMI) that is too low or too high are at increased risk for complications and additional hospitalization, according to a new Scottish study.

BMI is a measurement of body fat based on height and weight.

The findings -- published Sept. 18 in BJOG: An International Journal of Obstetrics and Gynaecology -- highlight the need for strategies to reduce obesity rates, said study co-author Dr. Fiona Denison of Queens's Medical Research Institute in Edinburgh.

"Longer-term benefits of reducing maternal obesity will show improvements, not only in the health outcomes of mothers and their babies, but the workload and cost to current maternity services," she said in a journal news release.

For the study, researchers examined data from nearly 110,000 pregnant women in Scotland who were divided into five groups based on BMI: underweight, BMI of less than 18.5; normal weight, BMI of 18.5 to 24.9; overweight, BMI of 25 to 29.9; obese, BMI of 30 to 35; and severely obese, BMI of more than 35.

The risk of pregnancy complications among women increased with BMI, according to the study.

Severely obese women were three times more likely than normal-weight women to have high blood pressure (7.8 percent vs. 2.6 percent) and gestational diabetes (3 percent vs. 0.1 percent).

Compared to normal-weight women, those in all other weight categories had more and longer hospitalizations after birth. The risk of hospitalization was 8 percent higher for underweight women, 16 percent higher for overweight women, 45 percent higher for obese women and 88 percent higher for severely obese women.

The study also found that women with high or low BMI had higher medical costs than normal-weight women.

More information

The U.S. National Institute of Child Health and Human Development has more about weight and pregnancy.