(1030) Do concurrent risk factors modify the effect of pre-pregnancy body-mass-index on adverse perinatal outcomes?

High pre-pregnancy body-mass-index (BMI) is one of the most common risk factors for adverse perinatal events. Our aim was to assess whether this effect of BMI on adverse perinatal outcomes is modified by other concomitant maternal risk factors.

Study Design:

This was a retrospective cohort study of all singleton births in the United States, 2016-2017, using data from the National Centre for Health Statistics. Logistic regression was used to estimate the adjusted odds ratios (aOR) between pre-pregnancy BMI and a composite outcome of stillbirth, neonatal death, and severe neonatal morbidity. Multiplicative and additive modification of this association by maternal age, parity, chronic hypertension and diabetes mellitus was assessed.

Results:

The study included 7,576,417 women; 254,225 (3.5%) were underweight, 3,220,432 (43.9%) had normal BMI, 1,918,480 (26.1%) were overweight and 1,062,177 (14.4%), 516,693 (7.0%), 365,357 (5.0%) had class I, II and III obesity, respectively. The composite outcome rate increased with increasing BMI. Nulliparity, hypertension and diabetes modified the association between BMI and the composite perinatal outcome. Nulliparous women had a higher rate of increase in adverse outcomes with increasing BMI (e.g., aOR 1.77, 95%, CI 1.73-1.83 for obesity class III) compared to multiparous (1.35 (95% CI 1.32-1.39). Women with chronic hypertension or diabetes had higher outcome rates overall; however, the dose-response relationship with increasing BMI was absent. While the composite outcome rates increased with maternal age, the risk curves were similar across obesity classes and  maternal age groups.

Conclusion:

Women with elevated pre-pregnancy BMI are at increased risk of adverse perinatal outcomes, and the magnitude of these risks is modified by concomitant risk factors. In women with chronic hypertension or diabetes, BMI does not increase risk of adverse outcomes, but the overall outcome rate remains high. Pre-pregnancy prevention of hypertension and diabetes should be emphasized among all women irrespective of BMI.