<unk>  is a placental morphological abnormality, a subtype of placenta extrachorialis in which the fetal membranes (chorion and amnion) "double back" on the fetal side around the edge of the placenta. After delivery, a<unk>  has a thick ring of membranes on its fetal surface. The fetal surface is divided into a central depressed zone surrounded by a thickened white ring which is incomplete. The ring is situated at varying distance from the margin of the placenta. The ring is composed of a double fold of amnion and chorion with degenerated decidua vera and fibrin in between. Vessels radiate from the cord insertion as far as the ring and then disappear from the view. Complete<unk>  occurs in approximately 1% of pregnancies. It is diagnosed prenatally by medical ultrasonography, although one 1997 study of prenatal ultrasounds found that "of the normal placentas, 35% were graded as probably or definitely<unk>  by at least one sonologist," and "all sonologists misgraded the case of complete<unk> on as normal." The condition is associated with perinatal complications such as placental abruption, oligohydramnios, abnormal cardiotocography, preterm birth, and miscarriage.
Circumvallate placenta
Retained placenta
Uterine septum
Wandering womb
Ectopic decidua
Placental expulsion
Prelabor rupture of membranes
Placenta praevia