ULTRASONIC ASSESSMENT OF THE AMNIOTIC-FLUID VOLUME IN DIAMNIOTIC TWINS

OBJECTIVE: We sought to determine amniotic fluid (AF) volume in diamni otic twin gestations and to relate these findings to estimates using s tandard ultrasonic techniques. METHODS: 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 sonogra phic estimations of AF volume, the true amount of AF was determined us ing amniocentesis and a dye-dilution technique. RESULTS: The individua l 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 volum e between 500 and 2000 mL was correctly predicted by sonographic use o f 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 move ac curate using the two-diameter pocket method compared with either the A F index (P =.015) or the largest vertical pocket technique (P <.0001). CONCLUSIONS: 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 s onographic method.