OBJECTIVE Our purpose was to determine the amniotic fluid volume in normal diamniotic twins. METHODS The single amniotic fluid index for both twin members, the maximum depth and width of each twin's largest pocket, were measured every 4 to 6 weeks between 15 and 40 weeks in 91 normal diamniotic twin gestations. The two-diameter pocket in each twin was determined as its largest pocket's depth multiplied by its width. Normal twin gestations were defined as those with <20% birth weight discordance, appropriate for gestational age, no fetal anomalies, delivery at > or = 37 weeks, and normal newborns. Amniotic fluid index values, depths, and two-diameter pockets were stratified into 2-week intervals and transformed into base 10 logarithms because of their nongaussian distributions. The correlations of these measurement with gestational age were evaluated. Their means and 90%, 95%, and 98% confidence intervals were determined. RESULTS The amniotic fluid index changed significantly with gestational age. However, depths and two-diameter pockets did not. The amniotic fluid index rose from 15 to 24 weeks, plateaued until 36 weeks, and then declined. The 90%, 95%, and 98% confidence intervals for each twin's depth were 2.4 to 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 were 8 to 44, 7 to 52, and 5 to 63 cm2 (mean 19 cm2). The amniotic fluid index confidence interval curves were plotted from the log (amniotic fluid index)=0.8276 + 0.01675x-0.0000001900x4, R2=0.78, p=0.002 (where x is gestational age). CONCLUSIONS The amniotic fluid volume in normal diamniotic twin pregnancies was established ultrasonographically. Only the amniotic fluid index changed significantly with gestational age. These findings may have significance in the clinical management of twin gestation.