They studied 13,442 pregnancies among women aged 18 and older who were Kaiser Permanente Northwest members between 2000 and 2004.
Data included how long the women stayed in the hospital when they gave birth, whether they gave birth vaginally or by C-section, how often they got health care during pregnancy, and whether they had pregnancy risks, including type 2 diabetes, gestational diabetes, and high blood pressure.
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.