1ST TRIMESTER 3-DIMENSIONAL ULTRASOUND VOLUMETRY OF THE GESTATIONAL SAC

First trimester amniotic fluid is an ultrafiltrate of maternal plasma and constitutes the major component of gestational sac volume (GSV). W e hypothesized that GSV, assessed by 3-dimensional (3-D) ultrasound vo lumetry, would reflect function of the early uteroplacental unit and t herefore provide a basis for predicting pregnancy outcome. We tested t his hypothesis in 38 pregnancies which had first trimester GSV measure ments by two investigators thus allowing determination of interobserve r variation. Gestational age (GA) was based on a careful history and c onventional 2-dimensional ultrasound measurements. Serum for beta-hCG, estradiol (E2) and progesterone (P) was obtained at the time of ultra sound examinations. ''Normal'' outcome was defined as confirmation of a viable fetus. ''Abnormal outcome'' was defined as either a ''blighte d ovum'' or embryonic demise. Statistical analysis was performed by In dependent t-test and regression analysis. There were 31 ''normal'' and 7 ''abnormal'' pregnancies studied between 5 and 11 weeks gestation ( mean +/- SD 8.3 +/- 1.3 weeks). GSV was significantly correlated to GA (r = 0.74, P < 0.001), higher than to beta-hCG (r = 0.40, P = 0.034), E2 (r = 0.70, P < 0.001) and P (r = 0.21, P = 0.334), respectively. I n the abnormal group 2/2 pregnancies with a twin sac had a GSV within 1 SD of the mean. 3/5 cases of missed abortions or blighted ovum had a GSV < 2 SD of the mean. The interobserver correlation was high (r = 0 .99, P < 0.001). This is the first clinical study investigating the di agnostic use of 3-D ultrasound volumetry in first trimester pregnancy. Our results suggest that GSV volumetry has the potential to predict p regnancy survival into the mid to late trimester.