Ultrasonic demonstration of the amnion.

The amnion can be visualized in most pregnancies before the 12th week. It appears as a thin membrane separating the amniotic cavity, which contains the fetus, from the extra-embryonic coelom and the secondary yolk sac. In a prospective study of 32 women in the first 12 weeks of pregnancy, the amnion was demonstrated ultrasonographically in 27 (84 per cent). The embryonic origin of the amnion and possible difficulties in its demonstration by ultrasound are described.