The prevalence of normal weight in women entering pregnancy dropped between 2011 and 2015 in all 38 jurisdictions with available data, according to the Centers for Disease Control and Prevention.

The overall prevalence of normal prepregnancy weight declined from 47.3% to 45.1% over that period in 36 states, the District of Columbia, and New York City, which reports natality data separately from New York state. The decreases were statistically significant in 26 states and New York City, the CDC investigators reported (MMWR. 2018 Jan 5;66[51-2]:1402-7).

“Corresponding with the decline in prepregnancy normal weight prevalence during 2011-2015, the entire [body mass index] distribution shifted toward a higher BMI,” they wrote. Along with an 8% drop in the prevalence of underweight status before pregnancy, overweight status rose by 2% and obesity status increased by 8%, with class III obesity (BMI 40.0 or greater) prevalence increasing by 14%, compared with 10% for class II obesity (BMI 35.0-39.9) and 6% for class I (BMI 30.0-34.9).

Based on data from 48 states, D.C., and New York City, the distribution of prevalence for the BMI categories in 2015 was 3.6% underweight, 45% normal weight, 25.8% overweight, and 25.6% obese, the investigators said.

The CDC analysis was based on natality data from the National Vital Statistics System. The standard birth certificate was revised in 2003 to include maternal height and prepregnancy weight, but only 38 jurisdictions were using it by 2011. By 2015, all states except Connecticut and New Jersey had adopted its use.