Ultrasonic assessment of the amniotic fluid volume in diamniotic twins.

Abstract

We sought to determine amniotic fluid (AF) volume in diamniotic twin gestations and to relate these findings to estimates using standard ultrasonic techniques. In this prospective study, AF volume in 45 diamniotic twin gestations at 27-38 weeks was assessed by sonography using the largest vertical pocket technique, the AF index, and the two-diameter pocket method. After the three different sonographic estimations of AF volume, the true amount of AF was determined using amniocentesis and a dye-dilution technique. The individual sac AF volume was less than 500 mL in 35 amniotic cavities, 500-2000 mL in 48 sacs, and greater than 2000 mL in seven. Individual AF volume between 500 and 2000 mL was correctly predicted by sonographic use of the largest vertical pocket in 47 of 48 patients (98%), the AF index in 47 of 48 (98%), and the two-diameter pocket method in 39 of 48 (81%). Estimation of AF volume less than 500 mL was significantly more accurate using the two-diameter pocket method compared with either the AF index (P = .015) or the largest vertical pocket technique (P < .0001). Currently available ultrasonic techniques to assess mid-range AF volume (500-2000 mL) in twin pregnancy are very accurate (81-98%). Oligohydramnios (less than 500 mL) is poorly identified by any sonographic method.