Abnormal membranes in obstetrical ultrasound: incidence and significance of amniotic sheets and circumvallate placenta.

Amniotic sheets were identified in ten of 1784 patients having routine obstetric ultrasound scans, giving an incidence of 0.6%. In the same population, findings suggesting circumvallate placenta were seen in three patients, giving an incidence of 0.2%. The overall frequency of finding abnormal membranes projecting into the gestational sac was 0.8% which is an order of magnitude greater than the incidence of amniotic bands, which are only present in up to 0.08% of pregnancies. It is important to distinguish amniotic sheets and circumvallate placenta from amniotic bands, since they are much more likely to be found during the ultrasound examination and need to be properly identified so as to avoid needlessly alarming pregnant patients or inappropriately considering termination of pregnancy for relatively benign entities. Copyright 1993 International Society of Ultrasound in Obstetrics and Gynecology