Pre-pregnancy Obesity Phenotypes and the Risk of Oligohydramnios: A Large Population-Based Cohort

Objective To evaluate the association between pre-pregnancy maternal obesity phenotypes—general, abdominal and combined—and the risk of oligohydramnios. Design Nationwide population-based cohort study. Setting South Korean National Health Insurance Service database. Population Women who underwent national health screening within two years before conception (2009–2017) and delivered between 2010 and 2018 (n=571,330). Methods Maternal obesity phenotypes were defined by pre-pregnancy body mass index (general obesity) and waist circumference (abdominal obesity). Oligohydramnios was identified using ICD-10 codes. Adjusted odds ratios (aORs) and 95% confidence intervals (CIs) were estimated using multivariable regression models controlling for maternal and pregnancy factors. Main outcome measures Risk of oligohydramnios according to pre-pregnancy maternal obesity phenotypes. Results Oligohydramnios occurred in 1.13% of pregnancies (n=6,484). Women with general obesity (BMI≥25kg/m2) had a higher risk compared with those with normal BMI (aOR 1.40, 95% CI 1.21–1.61). Abdominal obesity (WC ≥85cm) also increased risk (aOR 1.21, 95% CI 1.10–1.33). The combined phenotype conferred the highest risk (aOR 1.24, 95% CI 1.12–1.37), while either phenotype alone was not significant after adjustment. Conclusion Coexisting general and abdominal obesity before pregnancy independently increase the risk of oligohydramnios. Phenotype-based assessment may improve preconception risk stratification and guide targeted antenatal surveillance.