Circumvallate placenta, a form of placenta extrachorialis, should be included in the differential diagnosis of vaginal bleeding in the second trimester with a normally implanted placenta. Heretofore, the diagnosis was made after delivery. This report presents a case of circumvallate placenta diagnosed prenatally by ultrasound. Key features included an infolding of the fetal membrane upon the fetal surface of the placenta during the middle of the second trimester. By the third trimester, only a bright border at the periphery of the placenta was noted. Antenatal diagnosis can be made and pregnancy outcome potentially altered.