Higher Prepregnancy BMI Increases Rate of Infant Mortality

August 26, 2020

Higher prepregnancy BMI results in higher rates of infant mortality.

Increasing prepregnancy body mass index (BMI) leads to higher rates of infant, neonatal, and postneonatal mortality, according to a recent National Vital Statistics report.

The findings of the report may help inform research and prevention efforts on infant mortality related to maternal prepregnancy factors.

Danielle Ely, PhD, and investigators used data from the 2017-2018 linked birth/infant death period files. The data included all infant deaths reported from death certificates and live births reported from birth certificates. Data were based on 99.6% of the 22,341 infant deaths among US residents in 2017 and 99.3% of the 21,498 infant deaths in 2018. Evaluated data were based on the 2003 US Standard Certificate of Live Birth.

Mothers self-reported their prepregnancy weight and height at the time of delivery. Prepregnancy BMI was calculated by taking the mother’s weight before pregnancy and dividing it by their height in inches squared and multiplied by 703. The BMI categories were underweight (BMI under 18.5), normal weight (BMI of 18.5 to less than 25), overweight (BMI of 25 to less than 30), and obesity (BMI of 30 or more).

The report included data for births and infant deaths to Hispanic mothers and for white, black, AIAN, and Asian race groups.

During the study period, 3.3% of births were to underweight women while 42.7% were to normal-weight women. Among infants who died, 3.5% were born to women who were underweight and 35.8% were born to women of normal weight. More than half the births from 2017-2018 were to women who were overweight (26.4%) or had obesity (27.6%). Approximately 61% of infants who died were born to women who were overweight (25%) or had obesity (35.7%).

For infant mortality rates by maternal BMI, the lowest rate was 4.57 per 1000 births for infants born to women of normal weight. The rates rose for births to overweight women (5.16) and those who had obesity (7.07)—the rate for infants of women with obesity was 55% higher than that for infants of normal-weight women.

The lowest neonatal mortality rate was the lowest for normal-weight women. The rates increased with rising BMI from the normal to obese weight categories (2.91 for normal, 3.41 for overweight, and 4.76 for obese). The rate for infants born to women with obesity was 64% higher than the rate for normal weight.

The team found postnatal mortality rates were lowest for infants of normal weight (1.66) and increased through the obesity category (2.3). For women with obesity, the rate was 39% higher than for normal-weight women.

Mortality rates for infants of those under 30 years old with obesity were 27-52% higher than rates for infants of normal weight women under 30 years old. And the rates for infants of women older than 30 years old with obesity were 64-92% higher than rates for infants of normal weight women of the same age.

The mortality rates for infants of females under 20 years old were lowest for infants of normal weight females (7.82) and increased to 7.97 among overweight females and to 10.24 among females with obesity.

Among non-Hispanic white, non-Hispanic black, non-Hispanic Asian, and Hispanic women, mortality rates increased with rising maternal BMI from infants of normal weight women through those of women with obesity. The lowest rate of mortality for non-Hispanic white women was for those of normal weight (3.88), increasing to 4.22 among overweight women and to 5.66 among women with obesity.

Overall, mortality increased as maternal BMI increased from the normal weight class through obese weight. The findings showed the risk was consistently higher for infants of women with obesity compared to normal-weight women for each infant age at death group and across maternal age, maternal race, and Hispanic-origin groups. Such results could inform research and prevention efforts on infant mortality related to maternal prepregnancy factors.

The study, “Infant Mortality by Maternal Prepregnancy Body Mass Index: United States, 2017-2018,” was published by the National Center for Health Statistics.