Amniotic sheets: natural history and histology.

Eleven amniotic sheets were detected on obstetric ultrasound. Ten of these were reviewed retrospectively and one was followed prospectively throughout gestation. Amniotic sheets are single, planar reflective membranes. Evidence presented here suggests that these membranes are composed of four distinct layers: two layers of chorion sandwiched between two layers of amnion. Their mean thickness is 2.4 mm in the midportion and 4.5 mm at the free edge. A thick triangular base is frequently seen. Amniotic sheets change little during pregnancy; however, they are more difficult to identify late in gestation. They are unassociated with fetal anomalies. Mothers with amniotic sheets had a substantial incidence of previous spontaneous or therapeutic abortions.