Two closely applied but separate membranes line the uterine cavity and surround
the developing embryo-fetus. Both membranes, the amnion (inner membrane) and the
chorion (outer membrane), arise from the zygote. As the chorion develops, it blends
with the fetal portion of the placenta; the amnion blends with the fetal umbilical cord.
These deceptively strong, translucent membranes contain not only the fetus but also the
amniotic fluid, and they are continuous with the margins of the placenta. See figure 2-7.
Figure 2-7. Fetal membranes.
a. Amnion. This is the smooth, slippery, glistening innermost membrane that
lines the amniotic space. It is filled with fluid and is often called the "bag of water." The
fetus floats and moves in the amniotic cavity. At full term, this cavity normally contains
500 cc to 1000 cc of fluid (water). This fluid provides many functions for the fetus. The
amnion usually ruptures just before birth. The amnion functions to:
(1) Protect the fetus from direct trauma by distributing and equalizing any
impact the mother may receive.
Separate the fetus from the fetal membranes.
(3) Allow freedom of fetal movement and permits musculoskeletal
Facilitate symmetric growth and development of the fetus.
(5) Protect the fetus from the loss of heat and maintains a relative, constant
fetal body temperature.