Explanation of Amniotic Fluid, Its Function, and Disorders

By
Beth S, M.D.

Beth is a Doctor of Medicine who has worked in public health. She has diverse medical knowledge and experience in many areas.

Learn about our Editorial Policy.

Amniotic Fluid

A developing baby floats in a special kind of liquid called amniotic fluid. This fluid is the "water" that comes out when a woman's water breaks.The amniotic fluid cushions and protects the baby. It gives the baby room to move around and exercise his/her developing muscles. It also helps the baby's lungs develop properly. The baby "breathes" the liquid, which helps the lungs expand and grow.

The fluid is made by the placenta at first, and then by the baby. When the baby swallows it, it passes through the digestive system and into the bloodstream. It is processed through the baby's kidneys and passes back out of his/her body as urine. The urine then becomes more amniotic fluid.

After a baby is born, his/her urine will contain waste products from the body. But when the baby is still developing, a lot of those waste products are cleared from his/her blood as it passes through the placenta. So swallowing amniotic fluid, even though it's largely made of urine, does not hurt the baby.

Normal Amniotic Fluid

Amniotic fluid is monitored as part of regular prenatal care. It's measured during routine ultrasound exams, which also look at the baby's development and growth.

In a normal pregnancy, the amount of fluid increases as the baby grows. By 38 weeks, there should be about 800ml. The amount decreases a little after that. At full term, which is 40 weeks, there should be about 600ml.

Polyhydramnios

When there is too much amniotic fluid, the condition is called polyhydramnios. Polyhydramnios can be a sign of a multiple pregnancy. It can happen when the mother has gestational diabetes. It can also signal that something is wrong with the developing baby.

Polyhydramnios can happen when the baby is not able to swallow the liquid in normal amounts. It can also happen if too much fluid is being produced. Problems that can cause polyhydramnios include:

  • Birth defects involving the digestive system
  • Neurological defects
  • Down's syndrome and certain other genetic disorders

Oligohydramnios

Oligohydramnios is the medical term for too little amniotic fluid. This can be normal if the pregnancy goes beyond the due date, because the amount of fluid naturally declines as a pregnancy progresses. When it happens earlier in the pregnancy, it can indicate a birth defect. The baby needs a certain amount of fluid to move around and grow. When there is too little, the baby can become deformed due to compression by the uterus. Adequate fluid is also necessary for the baby's lungs to develop properly. Oligohydramnios can be caused by:

  • Problems with the baby's kidneys
  • Problems with the placenta
  • Leakage of fluid or premature rupture of membranes (the "water breaking" too early)
  • Some medicines, such as ACE-inhibitors (often used to treat high blood pressure in people who are not pregnant)

Amniocentesis

Amniocentesis is a prenatal test which involves taking a sample of amniotic fluid. Amniocentesis is not done for every pregnancy. It is usually offered if there is some reason to be concerned about the baby's development.Amniotic fluid contains cells from the baby's body. Doctors can use these cells to look at the baby's genetic material and diagnose a number of problems, including Down's syndrome. The fluid also contains chemicals produced by various processes in the baby's body. These chemicals can reveal whether the baby's lungs are mature enough to breathe outside the womb.

Amniocentesis maybe be offered, usually during the second trimester, if:

  • The triple screen is abnormal
  • The mother is older than 35, which increases her risk of having a baby with Down's syndrome
  • The mother has a previous child with a birth defect
  • Something else in the family history raises a concern

Later in the pregnancy, amniocentesis may be needed if:

  • The baby must be delivered prematurely
  • The baby develops Rh Disease
  • There appears to be an infection in the uterus

Amniocentesis carries a small risk of causing miscarriage. There is also a smaller risk of causing uterine infection after the procedure.

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Explanation of Amniotic Fluid, Its Function, and Disorders