The anechoic retroplacental area: a pitfall in diagnosis of placental--endometrial abnormalities during pregnancy.

A total of 116 consecutive obstetric ultrasound examinations were reviewed. An anechoic area was observed at the placental-endometrial junctionin all 54 fundal placentas, 51 of the 52 posterior placentas, and 9 of the 36 anterior placentas. Detail was enhanced by use of a focused transducer with digital gray-scale equipment, and visualization of anterior placentas was improved by placing a water bath between the transducer and the skin surface. The anechoic area was seen beginning in the twelfth week of gestation and corresponds best to the dilated vessels of the decidua basalis.