The placenta is an important part of every obstetric ultrasound examination, but its evaluation is often underemphasized. Important abnormalities of the placenta can be detected by ultrasound-some of them before they adversely affect the fetus. Most of these abnormalities are vascular and include subchorionic hemorrhage, intervillous thrombosis, and placental abruption. The authors present a placental vascular anomaly not described previously. A transiently filling and emptying retroplacental and subchorionic space, corresponding with venous flow, is described by the authors. Throughout an 18-month period, three cases were observed. The pregnancy outcomes were good in all cases, with healthy neonates at birth. The clinical course and pregnancy outcome of these three cases are described.