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.