Trimester-Specific Bioelectrical Impedance Reference Curves in Physiological Pregnancy: Mapping Maternal Body...

Background/Objectives: Clinical risk measures such as pre-pregnancy body mass index (BMI) and gestational weight gain do not distinguish the expansion of fluid, lean, and adipose compartments. Standardized obstetric reference curves for bioelectrical impedance analysis (BIA) are lacking. This study aimed to establish longitudinal, trimester-specific BIA reference curves in uncomplicated pregnancies. Methods: This multicenter, prospective, longitudinal cohort study was conducted at three Italian tertiary research hospitals from September 2020 to September 2025. BIA assessments (InBody 270^®) were performed at 10-15, 23-25, and 34-37 gestational weeks. Generalized additive models for location, scale, and shape were used to derive reference centiles and BMI-conditioned contour plots for total body water, lean mass, fat mass, and visceral fat level. Pregnancies with complications or incomplete longitudinal data were excluded from the primary reference cohort. Results: Among 442 healthy pregnant women, all parameters increased progressively across gestation. Each 1-unit increase in pre-pregnancy BMI was associated with a larger shift in fat mass (1.88 kg; 95% CI, 1.77-1.99) than in total body water (0.47 L; 95% CI, 0.38-0.57). Conclusions: These multicenter, BMI-conditioned, trimester-specific BIA reference curves provide normative benchmarks for maternal hydration and body composition and may support the identification of early deviations from physiological gestational trajectories.