Which fetal membrane is the outermost and is typically obliterated by the amnion between 12 and 16 weeks?

Which fetal membrane is the outermost and is typically obliterated by the amnion between 12 and 16 weeks?

• A. Chorion
• B. Amnion
• C. Yolk sac
• D. Allantois

Answer: (A)

The outermost fetal membranous layer is the chorion. It surrounds the developing embryo and lies just outside the amnion. The amnion forms inside and gradually expands, eventually enveloping and fusing with the chorion. This fusion typically occurs around 12 to 16 weeks, obliterating the space between them (the chorionic cavity) and creating the amniochorionic membrane that encases the fetus. The yolk sac and allantois are other fetal membranes with different roles and positions, and they do not become the outermost layer or get obliterated by the amnion in this way.

The outermost fetal membranous layer is the chorion. It surrounds the developing embryo and lies just outside the amnion. The amnion forms inside and gradually expands, eventually enveloping and fusing with the chorion. This fusion typically occurs around 12 to 16 weeks, obliterating the space between them (the chorionic cavity) and creating the amniochorionic membrane that encases the fetus. The yolk sac and allantois are other fetal membranes with different roles and positions, and they do not become the outermost layer or get obliterated by the amnion in this way.