Background: While the obstetric risks of maternal obesity are heavily documented, maternal body mass index (BMI) at the initial prenatal visit—the "booking BMI"—exhibits a complex, U-shaped relationship with perinatal outcomes. Both extremes of the weight spectrum present distinct pathological pathways that jeopardize pregnancy safety.

 High booking BMI is a well-established driver of metabolic and vascular complications, driving up rates of gestational diabetes, preeclampsia, and macrosomia.

 Low booking BMI (underweight) introduces quiet but severe risks, primarily driven by nutritional deficiencies and placental insufficiency, which strongly correlate with foetal growth restriction, low birth weight, and spontaneous preterm birth.

​Evaluating individual complications in isolation often dilutes the statistical power and clinical reality of these risks. Utilizing a composite maternal and neonatal adverse outcome capture provides a comprehensive, multi-dimensional assessment of the true perinatal burden. This study investigates the association across the full continuum of maternal booking BMI—with explicit focus on both underweight and obese cohorts—to refine early risk stratification and optimize targeted prenatal interventions.

Methodology: This institution-based, retrospective observational study was conducted from January 2023 to January 2024 at Medical College, Kolkata. A total of 300 patients admitted in Eden building, Medical College, Kolkata were evaluated in the study.