Amniotic Fluid

Amniotic fluid is the fluid that surrounds the fetus. Amniotic fluid is 98% water and 2% salts and cells from the baby. A pregnant woman carries about 500-1000 ml of amniotic fluid. Until the fetal kidneys started working during month four, amniotic fluid is made by the mother’s body. But after month 4, the foetus started to make his/her contribution to the amniotic fluid by urinating into it. Successful pregnancy requires the accumulation of significant amounts of water, both to support fetal growth and to allow for maternal physiologic changes.

So let’s say 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 of the respiratory, digestive and musculoskeletal systems.
  • Absorbs blows against the mother’s abdomen.
  • It has antibacterial properties that provide some protection from infection.
  • Serves as a back up of nutrients and fluids for the baby.
  • Protect the foetus from heat loss by helping to regulate the correct foetal body temperature.
  • Keeps the embryo from sticking to the placenta. Towards the outside, the amniotic cavity is delimited by the amniotic epithelium, the chorionic leave and the deciduas’ capsularis.

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.

Amniotic fluid is also important for the health of the mother. This fluid fills the whole womb, so as the foetus grows and gains weight, no pressure is exerted on the womb itself. If this fluid were not present, the growing foetus would weigh the uterus down and the counter-pressure exerted by the uterine walls would make the normal development of the foetus impossible.

If there is a single problem with the production of this fluid, with its continuous purification or the adjustment of its volume, the natural development of the foetus is impaired. For example, if the amount of amniotic fluid is less than required, or if it is not present at all, a series of abnormalities begins to appear. Limbs wither and become deformed, joints fuse, skin loosens and, because of pressure, the face is deformed. The most serious problem is that the development of the lungs is impeded and the baby dies immediately after birth.

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Lowermilk, D., Perry, S. (2007). Maternity & Women’s Health Care. 9th Edition. Mosby/Elsevier: St Louis, Missouri.

Underwood, M. A., Gilbert, Sherman M. P. (2005). Amniotic Fluid: Not Just Fetal Urine Anymore. Journal of Perinatology. 25, 341–348.

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