The Maximal Vertical Pocket Better Identifies Pregnancies with Polyhydramnios at Increased Risk of Severe Neonatal Morbidity than the Amniotic Fluid Index: A Retrospective Cohort Study

Background: Polyhydramnios is associated with adverse perinatal outcomes and is commonly assessed using either the amniotic fluid index (AFI) or the maximal vertical pocket (MVP). Although both methods are accepted in clinical practice, they frequently classify the same pregnancy differently, and their relative ability to predict adverse neonatal outcomes remains uncertain. Methods: We conducted a retrospective cohort study of pregnancies delivered at a tertiary university medical center between 2010 and 2025. Pregnancies with an AFI or MVP measurement within two weeks before delivery were eligible. Pregnancies complicated by diabetes mellitus or major fetal anomalies were excluded. Polyhydramnios severity was classified separately according to AFI and MVP criteria. The primary outcome was a composite of severe neonatal morbidity including neonatal intensive care unit admission, umbilical artery pH < 7.1, 5 min Apgar score < 7, or perinatal death. Separate multivariable logistic regression models were constructed for AFI- and MVP-based severity classifications. Results: Among 32,928 pregnancies, 3146 (9.6%) had polyhydramnios. Moderate-to-severe polyhydramnios defined by MVP was independently associated with severe neonatal morbidity (adjusted odds ratio [aOR] 2.47, 95% confidence interval [CI] 1.57–3.90; p < 0.001), whereas AFI-based severity classification was not (aOR 1.44, 95% CI 0.98–2.12; p = 0.07). Severe neonatal morbidity increased progressively with increasing MVP measurements. Mild polyhydramnios, regardless of the measurement method, was not associated with increased neonatal morbidity. Similar findings were observed in sensitivity analyses. Conclusions: MVP-based severity classification identified pregnancies at increased risk of severe neonatal morbidity, whereas AFI-based classification did not, at their conventional thresholds. These findings suggest that MVP may provide better risk stratification in pregnancies complicated by polyhydramnios and support prospective studies to validate its role in clinical management.