The association between polyhydramnios and preterm delivery

OBJECTIVE: To determine whether an association exists between the severity of polyhydramnios and the frequency of preterm delivery. METHODS: The study group consisted of 275 singleton pregnancies with polyhydramnios as defined by an amniotic fluid index (AFI) of at least 25 cm. Polyhydramnios was arbitrarily categorized into three groups by severity: mild (AFI 25-30 cm), moderate (AFI 30.1-35 cm), and severe (AFI greater than 35.1 cm). A delivery was considered preterm if it occurred before 37 weeks' gestation. RESULTS: Fifty-two of 275 (18.9%) women delivered before 37 weeks' gestation. Preterm delivery occurred in 37 of 199 (18.5%) cases with mild polyhydramnios, 12 of 55 (21.8%) with moderate polyhydramnios, and three of 21 (14.3%) with severe polyhydramnios (no statistically significant difference). Fetuses with congenital malformations (16 of 41, 39%) and those of diabetic mothers (ten of 45, 22.2%) had a significantly higher incidence of preterm delivery than did cases with unexplained polyhydramnios (24 of 190, 12.6%; P < .001). The prematurity rate in cases with idiopathic polyhydramnios was no higher than the overall rate for our hospital. CONCLUSION: The underlying cause of polyhydramnios, rather than the relative excess of amniotic fluid as defined by this study, appears to determine when preterm labor will occur.