Amniotic sac development: ultrasound features of early pregnancy--the double bleb sign.

The amniotic sac-embryo-yolk sac complex can be seen with ultrasonography (US) as two small blebs of almost equal size attached to the wall of the early gestational sac. We have called this the double bleb sign. Since the developing embryo and its cardiac pulsation are located between these two blebs, the size of an early embryo can be measured. The crown-rump length thus measured corresponds well to that described in embryologic textbooks, and an embryo as small as 2 mm (5 weeks 3 days) can be measured with use of this method. Three stages of development of the amniotic sac-embryo-yolk sac complex can be identified with US, and they are useful in dating the pregnancy. These three stages are 1, a simple double bleb, 2, a double bleb with a stalk, and 3, a double bleb with a separating yolk sac. As the amniotic sac enlarges, strands of echoes may be seen within the chorionic cavity and should be differentiated from amniotic bands, which may cause serious fetal abnormality. Separation of the amniotic from the chorionic membrane before the 13th week is normal but may be abnormal after the 17th week of gestation. Separation of the chorionic membrane from underlying decidua, however, is abnormal at any stage of pregnancy.