Prevalence and Trends in Prepregnancy Normal Weight — 48 States, New York City, and District of Columbia, 2011–2015

Overall and among most jurisdictions examined, the prevalence of prepregnancy normal weight is decreasing; this suggests movement away from the Healthy People 2020 objective for prepregnancy normal weight. For women of reproductive age, BMI screening during routine clinical visits provides opportunities to address underweight or obesity, promote normal weight upon entering pregnancy, and ultimately help optimize maternal and child health outcomes.

Using data from the revised birth certificate for 48 states, the District of Columbia (DC), and New York City (NYC), this analysis found that the overall prevalence of prepregnancy normal weight was 45.0% in 2015; prevalence ranged from 37.7% in Mississippi to 52.2% in DC. Among 36 states, DC, and NYC with available prepregnancy BMI data from 2011 to 2015, prevalence of normal weight declined from 47.3% to 45.1%; declines were observed in all jurisdictions but were statistically significant among 27 after standardizing to the 2011 national maternal age and race/ethnicity distribution.

Entering pregnancy outside a normal weight (body mass index [BMI] of 18.5–24.9 kg/m 2 ) is associated with adverse maternal and infant health outcomes; given these outcomes, Healthy People 2020 includes an objective to increase the proportion of women entering pregnancy with normal weight. Recent trends in national or jurisdiction-specific prevalence of prepregnancy normal weight have not been reported.

Women who enter pregnancy at a weight above or below normal weight, defined as a body mass index (BMI) of 18.5–24.9 (calculated as weight in kg/height in m2), are more likely to experience adverse pregnancy outcomes and to have infants who experience adverse health outcomes. For example, prepregnancy underweight (BMI <18.5) increases the risk for small-for-gestational-age births, whereas prepregnancy overweight (BMI 25.0–29.9) and obesity (BMI ≥30.0) increase risks for cesarean delivery, large-for-gestational-age births, and childhood obesity (1). Given these outcomes, Healthy People 2020 includes an objective to increase the proportion of women entering pregnancy with a normal weight from 52.5% in 2007 to 57.8% by 2020.* Because recent trends in prepregnancy normal weight have not been reported, CDC examined 2011–2015 National Vital Statistics System (NVSS) natality data, which included prepregnancy BMI. In 2015, for 48 states, the District of Columbia (DC), and New York City (NYC) combined, the prevalence of prepregnancy normal weight was 45.0%; prevalence ranged from 37.7% in Mississippi to 52.2% in DC. Among 38 jurisdictions with prepregnancy BMI data during 2011–2015, normal weight prevalence declined from 47.3% to 45.1%; declines were observed in all jurisdictions but were statistically significant for 27 jurisdictions after standardizing to the 2011 national maternal age and race/ethnicity distribution. Screening women’s BMI during routine clinical care provides opportunities to promote normal weight before entering pregnancy.

NVSS collects demographic and health information for live births in 50 states†and DC via the U.S. Standard Certificate of Live Birth (birth certificate), which was revised in 2003 to include maternal height and prepregnancy weight. Height and prepregnancy weight are self-reported or abstracted from medical records§ and are used by NVSS to calculate prepregnancy BMI. The revised birth certificate was used in 36 states, DC, and NYC by 2011 and was used in 48 states, DC, and NYC by 2015 (representing 83% and 97% of all live births in 2011 and 2015, respectively).¶ Births to U.S. resident mothers in states adopting the revised birth certificate by January 1 of each year were eligible for analyses (17,906,182 mothers, representing 90% of all U.S. births during 2011–2015).** From these records, those with missing BMI (732,052) were excluded, resulting in 17,174,130 records for analysis (96% of births eligible for this analysis).

Prepregnancy BMI was categorized as underweight (<18.5), normal weight (18.5–24.9), overweight (25.0–29.9), or obese (≥30.0); for some analyses, obesity was categorized as class I (BMI = 30.0–34.9), class II (35.0–39.9), or class III (≥40.0). Overall and jurisdiction-specific prevalences for each prepregnancy BMI category were estimated. Overall and jurisdiction-specific trends were estimated as the percentage-point difference in prepregnancy normal weight prevalence from 2011 to 2015 for 38 jurisdictions with available data; overall trends for each prepregnancy BMI category were also estimated as the percentage change from 2011 to 2015. Because prepregnancy BMI increases with maternal age and varies by maternal race/ethnicity (2), jurisdiction-specific differences were estimated after directly standardizing each year to the race/ethnicity and age distribution††of 2011 U.S. resident mothers to facilitate comparisons. Standardized, jurisdiction-specific differences were evaluated using the z-statistic; p<0.05 was considered statistically significant.

For 48 states, DC, and NYC in 2015, the overall prevalence of prepregnancy normal weight was 45.0%; prevalences ranged from 37.7% in Mississippi to 52.2% in DC (Table 1). Among 38 jurisdictions with prepregnancy BMI data from 2011 to 2015, prevalence of normal weight declined from 47.3% to 45.1%; after standardization, this represented a 1.9 percentage-point decline (p<0.05). Declines in prepregnancy normal weight were observed in all 38 jurisdictions, but were statistically significant in 27 jurisdictions; declines ranged from 1.0 percentage point (p = 0.01) in Wisconsin to 3.5 percentage points (p<0.001) in Delaware over the 5-year period (Table 1).

Corresponding with the decline in prepregnancy normal weight prevalence during 2011–2015, the entire BMI distribution shifted toward a higher BMI (Figure). Specifically, there was an 8% decrease in the prepregnancy underweight prevalence, while there were 2% and 8% increases in overweight and obesity, respectively. Notably, class III obesity prevalence increased more rapidly than did class I or class II obesity (increase of 14% [class III], compared with 10% [class II] and 6% [class I]).

In 2015, jurisdictions with the highest prepregnancy normal weight prevalence (DC, Massachusetts, NYC, and Utah) had the lowest obesity prevalence, whereas jurisdictions with lowest prepregnancy normal weight prevalence (Mississippi and West Virginia) had the highest obesity prevalence (Table 2). Although NYC had a relatively high prevalence of prepregnancy normal weight, it also had the highest prevalence of underweight. Notably, some states exhibited a double burden of higher prevalences of prepregnancy underweight and obesity (Arkansas, Kentucky, and West Virginia).