These are the sacs that contain the fetus and the amniotic fluid. The Chorion: Is a thick, rough, opaque and fragile membranecontinuous with the placenta at its edge. It lines the decidua vera of the uterine cavity. It is derived from chorion leave of the trophoblast and continuous with the chorionic plate. It ruptures easily and can be retained during the delivery of the placenta.
The Amnion: It forms the sac that contains the fetus, the amnioticfluids and the cord. It lies in contact with the chorion. It is smooth, translucent and tough. It is derived from the inner cell mass. It is thought to have a role in the formation of the amniotic fluids. The amnion is much stronger than the chorion and hardly retained. It can be stripped off up to the insertion of the cord.
The Amniotic Fluid: It is straw-colored fluid, alkaline in reaction.It is secreted from amniotic membranes, exudates from the decidua and placenta vessels and from fetal urine. The volume is 400-1,500mls in normal cases. It increases at the rate of about 30mls per week but decrease at term as the baby fills the uterine cavity. It reduced to about 1 litre near term (38wk). The reduction in volume may be partly due to the fetus swallowing it at term. It is most abundant in mid-trimester. It has the specific gravity of 1010, 99% water. The 1% solid matter is composed of lanugo, hair, epithelia cells, vernix caseosa, protein, glucose sodium, potassium and calcium. It has pH of 7.0 – 7.5.
Less than 300mls is regarded as Low volume – oligoh ydramnios More than 1,500mls is regarded as High volume – Pol yhydramnios
Functions of Amniotic Fluid
1. Provides protective medium for the fetus against injuries.
2. Acts as shock absorber
3. Equalizes the pressure by uterine contraction over the fetus and cord.
4. It permits free movement of the fetus in utero.
5. Maintains the temperature of the fetus.
6. It flushes the birth canal at and before the delivery of the baby
7. Provides nutritive material
8. Help impede the entering of bacterial into the uterus.
9. Aids effacement and dilatation where there is poor application of the presenting part.