Central Adiposity Indicators by Pre-Pregnancy BMI at the 23 kg/m2 Cut-Off in Two Independent Cohorts of Thai Women During Pregnancy and Postpartum: A Descriptive Longitudinal Study.

Objectives: Prospective anthropometric data on how women above and below the Asian-specific BMI ≥ 23 kg/m2 cut-off differ across the maternal continuum remain limited. Because these data were collected in 2017-2018, before Thailand's 2022 adoption of the threshold in antenatal care, this study describes central adiposity indicators in two independent cohorts of Thai women as a pre-implementation baseline. Methods: In this descriptive longitudinal dual-cohort study at a tertiary centre in Northern Thailand, 192 women formed two independent cohorts: Cohort A (pregnancy, from 12 weeks' gestation; n = 110) and Cohort B (delivery to 24 weeks postpartum; n = 82), with no woman in both. Women were stratified by pre-pregnancy BMI (<23 vs. ≥23 kg/m2). Waist-to-height ratio (WHtR, primary) and mid-upper arm circumference (MUAC, secondary) were measured at seven timepoints and validated against 3D body scanning (r = 0.92-0.99). Results: At 12 weeks' gestation, the ≥23 kg/m2 stratum had higher WHtR (0.58 vs. 0.49; d = 1.56) and MUAC (32.52 vs. 26.19 cm; d = 1.98; both p < 0.001). Postpartum, the <23 kg/m2 stratum returned to WHtR < 0.50 by Week 8, whereas the ≥23 kg/m2 stratum remained above 0.50 through 24 weeks (0.56 vs. 0.48; p < 0.001). Conclusions: The ≥23 kg/m2 stratum showed higher central adiposity indicators from early pregnancy and, in the separate postpartum cohort, values that remained above 0.50 through 24 weeks. Because the cohorts comprised different women, these patterns are descriptive and cannot demonstrate within-woman retention. They nonetheless suggest the cut-off may identify women who could benefit from earlier antenatal and extended postpartum nursing assessment, pending confirmation in within-woman studies.