Gestational Weight Loss and Restrictive Weight Gain in Maternal Obesity: A Target Trial Emulation

This study asks whether the current weight-gain guideline for pregnant people with obesity should be lowered, and whether the answer depends on how severe the obesity is. The guideline from the Institute of Medicine (IOM) recommends that all pregnant people with obesity gain 5.0 to 9.0 kg, regardless of obesity class. However, growing evidence suggests that people with Class II or III obesity may benefit from gaining less than this range-or even losing a small amount of weight-without harming the baby. A randomized trial to test this directly would be ethically and practically difficult, so this study uses a different approach: it reconstructs what such a trial would have shown, using nationwide U.S. birth records. The study uses 2024 U.S. birth certificate data for people with pre-pregnancy obesity who had a single live birth. It compares three weight-gain patterns during pregnancy: (1) net weight loss (less than 0 kg gained), (2) restrictive gain (weight gained, but below the dynamic, gestational-age-specific IOM lower bound), and (3) standard gain within the IOM-recommended range. The main questions are whether restrictive gain or weight loss, compared with standard gain, reduces the risk of hypertensive disorders of pregnancy (such as gestational hypertension and preeclampsia) while not increasing the risk of a baby born small for gestational age (SGA). The study also examines whether the balance of benefits and harms differs across Class I, Class II, and Class III obesity. This is an observational study, not a randomized trial. It does not enroll participants or provide treatment. All analyses use existing, de-identified public health data. Results are intended to inform whether future weight-gain guidelines should provide class-specific recommendations rather than a single range for all obesity classes.