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Obstetrics Simplified - Diaa M. EI-Mowafi

The Foetal Membranes

The chorion: is the outer membrane. It is in contact with the uterine wall. It is attached to the margins of the placenta.
Histologically, it is composed of 4 layers:

  • cellular layer
  • dense reticulum
  • pseudo-basement membrane
  • outer trophoblast.

The amnion: is a transparent greyish membrane which lines the chorion. It covers the foetal surface of the placenta and the umbilical cord. The amniotic sac contains the foetus swimming in the liquor amnii.
Histologically, it is composed of 5 layers:

  • cellular layer
  • basement membrane
  • compact layer
  • fibroblast layer
  • outer spongy layer adherent to the cellular layer of the chorion.

THE AMNIOTIC FLUID (THE LIQUOR AMNII)

Nature

  • It is a clear pale, slightly alkaline (pH 7.2) fluid.
  • It is about 400 ml at mid pregnancy, reaches about 1000 ml at 36-38 weeks then decreases later on to be scanty in post-term pregnancy.

Composition

  • Water (98-99%),
  • carbohydrates (glucose and fructose), proteins (albumin and globulins), lipids, hormones (oestrogen and progesterone), enzymes (alkaline phosphatase),
  • minerals (sodium, potassium and chloride),
  • suspended materials as vernix caseosa, lanugo hair, desquamated epithelial cells and meconium.

Circulation of amniotic Fluid

The amniotic fluid is not in a static state but is in a continuous turn over, 500 ml of it are replaced each hour.

Origin

  • Foetal:
    • Active secretion from the amniotic epithelium.
    • Transudation from the foetal circulation.
    • Foetal urine.
  • Maternal:
    • Transudation from maternal circulation.

The foetal origin contributes more in the production of the amniotic fluid.
Uptake of amniotic fluid is by absorption through the amnion to the maternal circulation and by foetal swallowing.

Functions

  • During pregnancy:
    • Protects the foetus against injury.
    • A medium for free foetal movement.
    • Maintains the foetal temperature.
    • Source for nutrition of the foetus.
    • A medium for foetal excretion.
  • During Labour:
    • The fore-bag of water helps the dilatation of the cervix during labour.
    • It acts as an antiseptic for the birth canal after rupture of the membranes.