Physiology of the amniotic fluid

Amniotic fluid is to be found in the amniotic cavity. It completely surrounds the embryo after the 4th week of pregnancy. In this way it insures freedom of movement for the embryo, space for development, absorbs blows, and keeps the embryo from sticking to the placenta. Towards the outside, the amniotic cavity is delimited by the amniotic epithelium, the chorion laeve and the decidua capsularis. This is the interface to the maternal compartment. The amniotic fluid is a clear, watery fluid that is filtered out of the maternal blood via the amniotic epithelium into the amniotic cavity. A large portion stems also from the fetus itself (from the skin, the umbilical cord, the lungs and the kidneys). The makeup of the amniotic fluid is thus quite complex, with many maternal and fetal constituents.

The main constituents are water and electrolytes (99%) together with glucose, lipids from the fetal lungs, proteins with bactericide properties and flaked-off fetal epithelium cells (they make a prenatal diagnosis of the infantile karyotype possible).

Its quantity changes over the course of the pregnancy (20 ml in the 7th week, 600 ml in the 25th week, 1000 ml in the 30th to 34th week and 800 ml at birth). From the 5th month the fetus also begins to drink amniotic fluid (400 ml/day). Close to the end of the pregnancy the amniotic fluid is replaced all 3 hours, stressing the importance of this exchange between the amniotic fluid and the maternal compartment.

Pathology of the amniotic fluid

The amniotic bands are membranes that form in the amniotic fluid and can bind up limbs, possibly leading to amputations of the limbs or fingers and to cranio-facial abnormalities. The origin of these membranes is brought into connection with infections of the fetus or of the amniotic covering.

Fig. bride - Amniotic band

© Institut de pathologie, CHUV, Lausanne

Fig. bride