EXTRA-AMNIOTIC PREGNANCY

Sonographic characteristic of membrane	Relationship to Fetus / Placenta	Timing / Etiology
* Ruptured amnion retracts around base of cord from which it remains firmly attached. * Sheets of membranes floating freely in amniotic fluid. * Free edge present due to ruptured membrane.	* Fetus moves freely as there is no entanglement with amnion. * Fetus grows in the extraembryonic coelom. * No fetal constrictions or amputations.	* Timing unknown but must occur before amnion fuses with chorion. * Etiology obscure.

EXTRAMEMBRANOUS PREGNANCY

Sonographic characteristic of membrane	Relationship to Fetus / Placenta	Timing / Pathology
* Yet to be diagnosed antenatally. * Visualized free edge of amnion and chorion where hole is present. * Severe oligohydramnios.	* Fetus outside membranes within endometrial cavity. * Chronic leak of amniotic fluid. * Severe oligohydramnios:   - lung hypoplasia   - growth retardation   - joint dislocation   - clubfoot. * Typically circumvallate placenta with short umbilical cord.	* The size of the hole that the fetus escapes from suggests that it occurs early in the pregnancy. * Cause of rupture unknown. * Rupture of amnion and chorion. * Fetus migrates into endometrial cavity with no further growth of membranes