Adverse Perinatal Outcomes in Term Pregnancies Complicated by Borderline Oligohydramnios: A Retrospective Cohort Study

Abstract

The objective of this analysis was to assess whether adverse pregnancy outcomes at term differ between pregnancies categorised by ultrasound amniotic fluid volume as borderline low, low, or average. This study was designed as a retrospective cohort study during 2021-2025, comparing singletons born at term (37–42 weeks) whose mothers had an ultrasound evaluation of their amniotic fluid volume during the final week before birth. The investigators identified those whose ultrasounds indicated borderline amniotic fluid volume (5.1–8.0 cm AFI), low amniotic fluid volume (≤5.0 cm AFI), and those whose ultrasounds showed them to have a normal amniotic fluid volume (8.1–25.0 cm AFI). The investigators then randomly selected controls from the last group (those with ultrasounds indicating a normal amniotic fluid volume) to match each case in the first two groups. Cases with ultrasounds before the third trimester, cases with rupture of membranes after 32 weeks gestation, cases with multiple fetuses, and cases with a major maternal or fetal condition were eliminated. The main outcomes were small-for-gestational-age (SGA) infants and caesareans performed due to fetal distress. Additionally, a composite measure of adverse neonatal outcome was analysed. Multivariate logistic regression models were used to examine associations. A total of 622 pregnant women were studied (142 borderline oligohydramnios, 338 oligohydramnios, 142 controls). SGA neonates occurred more frequently in both the borderline and oligohydramnios groups compared to controls (11.3% and 11.2% respectively, vs 3.5%, p = 0.022), as well as caesarean deliveries for fetal distress (16.2% and 13.0% respectively, vs 4.9%, p = 0.009) and the combined measure (31.7% and 26.9% respectively, vs 16.9%, p = 0.013). Borderline oligohydramnios was independently associated with an SGA neonate (adjusted OR [aOR] 3.92, 95% CI = 1.35–11.38; p = 0.012) and with caesarean delivery for fetal distress (aOR 3.55, 95% CI = 1.34–9.42; p = 0.011). Rates of meconium-stained amniotic fluid, chorioamnionitis, overall caesarean delivery, and NICU admission did not differ significantly. Borderline oligohydramnios at term is associated with an increased risk of an SGA neonate and of caesarean delivery for fetal distress, approaching the risk seen with overt oligohydramnios. These pregnancies may warrant heightened fetal surveillance.