AbstractWe have identified six cases of chorioamniotic separation, three diagnosed in the third trimester and three cases seen in early pregnancy, at or prior to 16 wk' gestation. The sonographic findings consist of a fine mobile echodense line separated from the chorion and uterine wall by a lucent space. The membrane may completely surround the fetus and may extend over the fetal surface of the placenta, in which case the chorion may be identified as a separate echo. Two patients with chorioamniotic separation in the third trimester had polyhydramnios. The infants had no manifestations of amniotic band syndrome. The membranes seen in four other conditions are described and compared with chorioamniotic separation.