Lower Risk for Severe Morbidity, Mortality With Normal Prepregnancy BMI

A BMI chart on a table
A BMI chart on a table
Underweight woman were more likely to receive lifesaving interventions for antepartum hemorrhage and acute renal failure than women with normal BMI prepregnancy.

Underweight and overweight pregnant women have a higher risk for severe maternal morbidity and mortality compared with pregnant women who have a normal body mass index (BMI), according to a retrospective population-based cohort study published in the Journal of the American Medical Association.

A total of 743,630 women with a mean age of 28.1 [SD, 6.0] were included in the final analysis. Using BMI, investigators classified participants as underweight (3.2%), normal weight (47.5%), overweight (25.8%), obesity class 1 (13.1%), obesity class 2 (6.2%), and obesity class 3 (4.2%).

For underweight, normal weight, overweight, obesity class 1, obesity class 2, and obesity class 3 participants, the rates of severe maternal morbidity and mortality per 10,000 women were 171.5, 143.2, 160.4, 167.9, 178.3, and 202.9, respectively.

Compared with women who had normal BMI, those who were underweight, overweight, or obese had significantly higher rates of severe maternal morbidity and mortality, as demonstrated by multivariable analysis (adjusted odds ratio, 1.2 [95% CI, 1.0-1.3] for underweight participants, 1.1 [95% CI, 1.1-1.2] for overweight participants, 1.1 [95% CI, 1.1-1.2] for participants with class 1 obesity, 1.2 [95% CI, 1.1-1.3] for participants with class 2 obesity, and 1.4 [95% CI, 1.3-1.5] for participants with class 3 obesity).

The observational nature of this study prevented the researchers from discovering causal associations between weight and severe morbidity and mortality. Additionally, the investigators note that there was insufficient statistical power to evaluate correlations between BMI and rare morbidity and mortality subtypes in this sample.

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Although the findings did demonstrate greater rates of severe morbidity and mortality among the underweight, overweight, and obesity groups compared with normal-weight participants, “the absolute risk of severe maternal morbidity or mortality was small.”


Lisonkova S, Muraca GM, Potts J, et al. Association between prepregnancy body mass index and severe maternal morbidity. JAMA. 2017;318:1777-1786.