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What Is the Amniotic Fluid Index?

Ultrasounds are used to measure amniotic fluid index.
A baby's development requires the correct amount of amniotic fluid.
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  • Written By: Tricia Ellis-Christensen
  • Edited By: O. Wallace
  • Last Modified Date: 22 November 2014
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The amniotic fluid index (AFI) is a measurement and scale of the amount of amniotic fluid present in the womb while a woman is pregnant. Typically, the AFI calculation is determined as part of routine ultrasound examinations, somewhere around the 20th week of pregnancy or a little later. Interest in fluid levels might occur earlier if the measurement is likely to be too high or low, and, conversely, some women do not ever have their fluid levels measured if they don’t have an ultrasound. The measurement and its comparison to the index can be important in helping to determine fetal and maternal health and in making certain that just the right amount of fluid is present to support fetal development.

In the test to determine the amount of amniotic fluid, the sonographer or ultrasound specialist divides the uterus into four quadrants and measures the volume of fluid in each one with an ultrasound machine. The machine then calculates total level of fluid volume that exists in the quadrants, arriving at a number in centimeters that can say whether the levels are problematic. "Problematic" is defined as having too much fluid or too little, which are also called polyhydramnios and oligohydramnios, respectively.

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Polyhydramnios is diagnosed if a woman’s amniotic fluid index is 24 centimeters or greater. Oligohydramnios is diagnosed if AFI is 5 centimeters or less. Women with a measure that falls near either of these numbers, such as 6 or 23, might be carefully observed and reassessed at a later point.

The importance of rating pregnant women against the index is due to the problems associated with high and low levels of amniotic fluid that may complicate a birth or cause significant problems in a baby’s development. Low fluid levels may suggest maternal health issues, like gestational diabetes or autoimmune conditions like lupus. They’re also associated with some fetal birth defects, especially of the kidneys, and higher incidence of troubles at labor, including premature labor and higher risk for a baby being oxygen-deprived during the birth process.

When a woman has an amniotic fluid index measurement that is too high, this is suggestive of certain forms of birth defects, especially those that may affect a child’s ability to swallow liquids. Other causes happen more rarely but include an incompatibility between the Rh factors in the blood of mother and child or situations where twins have abnormal communications that threatens the health of one twin, which is called twin-to-twin transfusion.

The AFI isn’t always indication to do anything except watch carefully, and many women who have a low or high measurement have safe labors and healthy babies. When healthcare professionals intervene, they may decide to add fluid if AFI is low, or drain it when its is too high. Other interventions might be suggested based on the findings of ultrasounds. For example, a suspicion of certain birth defects might warrant more extensive ultrasounds, exams, or chromosomal testing to prenatally diagnose them.

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