Amniotic Fluid Index (AFI)
Definition - What does Amniotic Fluid Index (AFI) mean?
Amniotic fluid index (AFI) is the estimate of the volume of the fluid occupying the bag of waters that surround the unborn baby inside the uterus (called as the amniotic fluid) of a pregnant woman. This routinely used index of fetal well-being uses ultrasound to measure the amount of amniotic fluid. The normal range for the AFI is 5-18 cms. Reduced AFI levels (of less than 5 cm or less than the fifth percentile for the gestational age) hint at oligohydramnios, whereas raised AFI values (of more than 25 cm) are indicative of polyhydramnios.
FertilitySmarts explains Amniotic Fluid Index (AFI)
In order to calculate the AFI, doctors use the four-quadrant technique in which the uterus is divided into four imaginary quadrants. The maximal depth of fluid pockets devoid of the umbilical cord or fetal extremities in four abdominal quadrants is measured in the vertical dimension and the sum of the deepest pockets in all the four quadrants gives the AFI. Based on the AFI, the diagnosis of oligohydramnios is suspected when:
- An AFI indicates a fluid level of less than 5 cm (or less than the 5th percentile)
- The lack of a fluid pocket of 2-3 cm, or
- A fluid volume of less than 500 mL at second and third trimesters
Alternatively, physicians may use a "single deepest pocket (SDP)" technique to assess the amount of amniotic fluid. This technique involves measuring the single maximal deepest vertical pocket of amniotic fluid. The following is the significance of different levels of SDP:
- An SDP of 2-8 cm is normal
- A pocket of less than 2 cm hints at oligohydramnios.
- An SDP >8 cm suggests polyhydramnios
AFI is a less reliable tool than SDP for the assessment of the amniotic fluid volume in oligohydramnios. This is because measuring an AFI can result in too many false positives (i.e. lots of people are diagnosed with too little amniotic fluid, when, in fact, the amniotic fluid levels are normal. This results in greater obstetric interventions and delivery before term when compared with labeling oligohydramnios based on an SDP of less than 2 cm. Secondly, measuring the SDP is much easier than measuring deepest vertical pockets from all four quadrants of the uterus and summing them up. Moreover, SDP is a better tool for measuring the amniotic fluid volume in twin pregnancies in contrast to AFI.