The Amnion and Amniotic Cavity
Sonographic Characteristics of the membrane	Relationship to Fetus	Relationship to Placenta
* Thin membrane(0.2-0.5mm) * Only visualized when perpendicular to beam. * Seen in 80% of cases * Intimate relationship to fetus may obscure visualization. * Visualization is also dependent on the presence of amniotic fluid between amnion and fetus (first trimester oligohydramnios).	* Continuous with fetal skin at umbilicus. * Cannot be separated from its site of insertion on the umbilical cord. * Protects fetus from coming into contact with the external surface of the amnion and chorion.	* Covers the umbilical cord where it joins the placenta * Continuous with the placental edge * Cannot be separated from the fetal surface of the placenta

Timing

Pathology / Etiology

* Not visualized as a separate structure until the seventh week of gestation. *The amniotic cavity progressively increases in size, obliterating the chorionic cavity by 16-18 weeks post fertilization

* Separates the amniotic cavity containing the fetus, from the extra-embryonic cavity and the secondary yolk sac. * Consists of five tissue layers( one epithelium and four connective tissue layers). * Epithelial layer is squamous in nature, devoid of secretory ability, and resides on a basement membrane which is attached to the connective tissue layers. * Juxtaposition of amnion (connective layer) and chorion at 12 weeks, forming a potential space

The Chorion And Chorionic Cavity
Sonographic characteristics of the membrane	Relationship to fetus	Relationship to placenta
* Usually an inapparent membrane * Closely applied to uterine wall * Usually only visualized when a subchorionic hemorrhage separates it from the uterine wall. * The chorion and placenta have identical origins and are inseparably fused.	* There is no direct contact with the fetus unless a rupture of the amnion is present.	* Chorion always leads to the edge of the placenta * Cannot be separated from fetal surface of placenta but can be separated from endometrial lining (decidua parietalis) to which it is apparently fused.
Subchorionic Hemorrhage – arrow delineates attachment of chorion. P - placenta

Timing	Pathology / Etiology
* Fusion with the amnion at 14-16 weeks gestational age.	* The chorionic membrane (forms from chorionic laeve) and the fetal placenta (forms from the chorion frondosum) develop from the same embryonic cell layer and are therefore firmly attached to each other at the edges.

Chorionic fluid more echogenic than amniotic fluid

This is due to its higher protein content.