AFI is the score (expressed in cm) given to the amount of amniotic fluid seen on ultrasonography (aka ultrasound) of a pregnant uterus. To determine the AFI, doctors may use a four-quadrant technique, when the deepest, unobstructed, vertical length of each pocket of fluid is measured in each quadrant and then added up to the others, or the so-called "Single Deepest Pocket" technique.
An AFI between 8-18 is considered normal. Median AFI level is approximately 14 from week 20 to week 35, when the amniotic fluid begins to reduce in preparation for birth.
An AFI < 5-6 is considered as Oligohydramnios The exact number can vary by gestational age. The fifth percentile for gestational age is sometimes used as a cutoff value.