The CDC's Susan Chug, PhD, grouped women based on BMI (body mass index), which relates height to weight, before pregnancy or in early pregnancy.
Extra pounds were tied to longer hospital stays for childbirth. Compared with women of normal BMI, obese women stayed about half a day longer and extremely obese women stayed about one extra day in the hospital.
Overweight and obese women also tended to have more medical appointments during their pregnancies.
Those patterns were due to the fact that obese pregnant women were more likely than other women to have high-risk pregnancies and to have C-sections.
The findings, published in The New England Journal of Medicine's April 3 edition, may not apply to all pregnant women nationwide.