ULTRASONOGRAPHIC MEASUREMENT OF AMNIOTIC-FLUID VOLUME IN NORMAL DIAMNIOTIC TWIN PREGNANCIES

OBJECTIVE: Our purpose was to determine the amniotic fluid volume in n ormal diamniotic twins. STUDY DESIGN: The single amniotic fluid index for both twin members, the maximum depth and width of each twin's larg est pocket, were measured every 4 to 6 weeks between 15 and 40 weeks i n 91 normal diamniotic twin gestations. The two-diameter pocket in eac h twin was determined as its largest pocket's depth multiplied by its width. Normal twin gestations were defined as those with <20% birth we ight discordance, appropriate for gestational age, no fetal anomalies, delivery at greater than or equal to 37 weeks, and normal newborns. A mniotic fluid index values, depths, and two-diameter pockets were stra tified into 2-week intervals and transformed into base 10 logarithms b ecause of their nongaussian distributions. The correlations of these m easurements with gestational age were evaluated. Their means and 90%, 95%, and 98% confidence intervals were determined. RESULTS: The amniot ic fluid index changed significantly with gestational age. However, de pths and two-diameter pockets did not. The amniotic fluid index rose f rom 15 to 24 weeks, plateaued until 36 weeks, and then declined. The 9 0%, 95%, and 98% confidence intervals for each twin's depth were 2.4 t o 7.9, 2.1 to 8.8, and 1.9 to 10 cm, respectively (mean 4.3 cm). These respective confidence intervals for each twin's two-diameter pocket w ere 8 to 44, 7 to 52, and 5 to 63 cm(2) (mean 19 cm(2)). The amniotic fluid index confidence interval curves were plotted from the log (amni otic fluid index) = 0.8276 + 0.01675x - 0.0000001900x(4), R(2) = 0.78, P = 0.002 (where x is gestational age). CONCLUSIONS: The amniotic flu id volume in normal diamniotic twin pregnancies was established ultras onographically. Only the amniotic fluid index changed significantly wi th gestational age. These findings may have significance in the clinic al management of twin gestations.