OGTT diagnostic patterns, prepregnancy BMI, and residual risk of term macrosomia at routine 24–28-week screening: a retrospective cohort study

Routine oral glucose tolerance testing (OGTT) provides metabolic information related to fetal overgrowth, but results are often collapsed into a binary gestational diabetes mellitus (GDM) label. This may obscure residual risk among women with normal OGTT results and heterogeneity among those with abnormal results. We evaluated whether OGTT diagnostic patterns combined with prepregnancy body mass index (BMI) could describe residual macrosomia risk under usual care. This single-centre retrospective cohort included 13,560 term singleton live births in 2023–2024. OGTT patterns were classified as normal, isolated post-load abnormality (P-only), isolated fasting abnormality (F-only), or combined fasting and post-load abnormality (F + P). Macrosomia was birthweight ≥ 4,000 g. Models adjusted for advanced maternal age, BMI category, and parity; large for gestational age (LGA) was a supplementary outcome. Among 13,560 newborns, 394 had macrosomia (2.9%). Normal OGTT accounted for 80.4% of the cohort and had a macrosomia prevalence of 2.7%, whereas GDM accounted for 19.6% and had a prevalence of 3.8%. P-only, F-only, and F + P represented 8.6%, 6.2%, and 4.8% of the cohort, with prevalences of 2.7%, 3.4%, and 6.1%, respectively. Among women with normal OGTT results, prevalence increased from 0.9% in underweight women to 6.0% in women with obesity, and this BMI-related gradient persisted after adjustment. Among abnormal OGTT phenotypes, F + P remained associated with macrosomia (OR 1.64, 95% CI 1.15–2.33; P = 0.006). The four-category view exposed heterogeneity hidden by the binary GDM label but added little to discrimination. Supplementary LGA analyses showed increased risk for F-only and F + P. A normal OGTT result did not indicate uniformly low macrosomia risk; within this group, prepregnancy BMI further differentiated risk. Among abnormal OGTT phenotypes, F + P was associated with higher macrosomia risk under usual care. Considering OGTT patterns together with prepregnancy BMI may serve as a simple descriptive framework for characterising mid-pregnancy risk differences and for evaluating staged reassessment later in pregnancy. This framework is not yet supported for individual risk prediction or direct clinical management and requires external validation and prospective evaluation.