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What is amniotic fluid?

Asked by Giulia Rossi, age 13, of Rome, Italy

As any baby could tell you (if they could talk), amniotic fluid is the clear, yellowish liquid that surrounds the unborn baby. It is contained in the amniotic sac. The fluid consists mostly of fetal urine that bathes the developing fetus. The fetus floats in this fluid, which is warmed to the mother’s body temperature.

The amniotic fluid increases in volume as the fetus grows. The amount of amniotic fluid is greatest at about 34 weeks after conception or 34 weeks ga (gestational age). At 34 weeks ga, the amount of amniotic fluid is about 800 ml. This amount reduces to about 600 ml at 40 weeks ga when the baby is born.

Amniotic fluid is continually being swallowed and "inhaled" and replaced through being "exhaled", as well as being urinated by the baby. Not terribly tasteful to us in more ways than one. But the developing baby doesn't mind at all. It is essential that the amniotic fluid be breathed into the lungs by the fetus in order for the lungs to develop normally.

Analysis of amniotic fluid, drawn out of the mother's abdomen in an amniocentesis procedure, can reveal many aspects of the baby's genetic health. This is because the fluid also contains fetal cells which can be examined for genetic defects.

Amniotic fluid also protects the developing baby by cushioning against blows to the mother's abdomen, allows for easier fetal movement, promotes muscular/skeletal development, and helps protect the fetus from heat loss.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au