Evolution of the amniotic sac and extracelomic space as seen by early ultrasound examination

Amniotic sac and extracelomic space changes occurring from 5 to 14 weeks of pregnancy were observed with transvaginal ultrasound to gain new insights into their normal relationships during this time period.

Ninety-seven women from 5 to 15 weeks' gestation were enrolled in the study. Gestational age was determined by measuring embryonic crown-rump length. The embryo, amniotic sac, and gestational sac areas were measured using the best-fitted computer-generated elliptical view. The embryonic heart rate was measured using the M-mode function of the ultrasound equipment. Associations were determined using the least-squares method and Pearson's correlation coefficient.

Mean (range) gestational age calculated from CRL was 9.6 (5.3-14.6) weeks, embryonic heart rate 153 (100-188) beats per minute, embryonic surface 699.6 (5-2,199) mm(2), amniotic sac surface 1,383 (5-5,335) mm(2) and gestational sac surface was 1,517 (110-5,335) mm(2). Significant correlations between gestational sac surface, amniotic sac surface, embryonic surface, heart rate and gestational age were noted (p < 0.01). Changes in the amniotic sac surface correlated with embryonic surface but not with heart rate even when multiple regression analysis was attempted.

These data describe the normal relationships between the embryonic, amniotic sac, extracelomic space, and gestational sac surface, suggesting that increases in embryonic surface area is a determining factor for early expansion of the amniotic sac at these gestational ages.