Pre-pregnancy BMI and hypertensive disorders of pregnancy in Japan: population burden and simulated BMI reduction scenarios for preconception and interpregnancy care.

Hypertensive disorders of pregnancy (HDP) is a major cause of maternal morbidity worldwide. Although a higher pre-pregnancy body mass index (BMI) increases HDP risk, how HDP cases are distributed across BMI levels in relatively lean populations remains poorly characterized. We evaluated the association between pre-pregnancy BMI and HDP, the distribution of HDP cases across BMI levels, and model-predicted HDP incidence under hypothetical BMI distributions.This large-scale multicenter, retrospective study used longitudinal records of singleton pregnancies from 12 obstetric facilities in Japan between 2009 and 2024. The BMI-HDP association was modeled using parity-stratified spline logistic regression. Model-predicted HDP incidence under hypothetical BMI distributions was estimated using spline-based standardization with woman-level bootstrap resampling.Among 52,042 pregnancies, 2,592 (5.0%) developed HDP. Although nulliparous women had higher baseline risk than multiparous women, the BMI-associated risk gradient was similar across parity groups. Most pregnancies were within normal BMI ranges, and most HDP cases arose in women without overweight or obesity. A population-wide 0.5 kg/m² downward BMI shift yielded a 5.5% relative reduction in model-predicted HDP incidence, compared with 6.7% under a BMI cap of 25.0 kg/m². A population-wide 1.0 kg/m² downward shift yielded a 10.2% relative reduction, compared with 12.1% under a BMI cap of 23.0 kg/m².Pre-pregnancy BMI was associated with HDP risk across the BMI continuum, while many HDP cases occurred within moderate BMI ranges. Modest downward BMI shifts yielded model-predicted HDP incidence similar to selected BMI-capping scenarios. These findings provide quantitative benchmarks for population-wide and high-risk approaches to preconception weight management.