Objective: To define the normal range of amniotic fluid (AF) volume in third-trimester diamniotic twin gestations.
Methods: The AF volume of third-trimester diamniotic twin pregnancies was measured prospectively. An AF sample was drawn from both amniotic cavities of diamniotic twins either for diagnosis of subclinical chorioamnionitis with preterm labor and/or assessment of fetal lung maturity. Actual AF volume was determined in each amniotic cavity by a dye-dilution technique.
Results: The AF volume of 45 diamniotic twin pregnancies of 27-38 weeks' gestation was assessed. The AF volume per amniotic sac ranged from 155-5430 mL, with a mean (+/- standard deviation) of 877 +/- 860 mL. The AF volume in individual amniotic cavities and the total of both cavities in a diamniotic twin pregnancy were unchanged across the entire gestational age range (27-38 weeks). Neither fetal positioning within the uterus (lower twin versus upper twin) nor fetal weight were determining factors in the AF volume variability (P = .846 and P = .726, respectively). Based on absolute volumetric differences between the larger- and smaller-sac AF volume, we observed that as the AF volume of the larger sacs increased proportionately (P < .001), the AF volume of the smaller sacs remained relatively constant across the entire range of differences (P = .687) between 27-38 weeks.
Conclusions: The average AF volume of 877 mL for each diamniotic twin is similar to the AF volume reported for third-trimester singletons. The AF volumes of diamniotic twin gestations relate neither to fetal position nor fetal weight, but instead develop independently without relation to the other amniotic cavity. Ninety percent of the AF volumes were 215-2500 mL.