What is AFI (Amniotic Fluid Index)?
The Amniotic Fluid Index (AFI) is an ultrasound measurement technique that sums the deepest vertical pocket of amniotic fluid in each of the four quadrants of the uterus, providing a quasi-quantitative assessment of amniotic fluid volume during pregnancy. 1
How AFI is Measured
The AFI technique divides the uterus into four quadrants and measures the single deepest vertical pocket in each quadrant using ultrasound. These four measurements are then added together to produce the AFI value in centimeters 1. This differs from the maximum vertical pocket (MVP) method, which measures only the single largest pocket anywhere in the uterus 1.
Normal Values Throughout Pregnancy
AFI values vary with gestational age:
- The AFI rises progressively from early pregnancy through approximately 26-30 weeks of gestation, reaching a median peak of around 14.0-16.2 cm 1, 2, 3
- After 30 weeks, the AFI gradually declines until term 2, 3
- At 37 weeks or later, an AFI ≥5 cm is commonly used to define normal amniotic fluid 1
- Before 37 weeks, an AFI ≥8 cm is typically considered normal 1
- By 40 weeks, the median AFI declines to approximately 9.1 cm 3
Defining abnormal values:
Clinical Significance
AFI is not an independent test of fetal well-being but serves as an important component of fetal surveillance 1. Abnormal amniotic fluid levels can signify poor fetal health:
- Oligohydramnios may be associated with intrauterine growth restriction (IUGR) and is an independent risk factor for stillbirth (odds ratio 2.6) 1
- Polyhydramnios may indicate maternal gestational diabetes and is also independently associated with stillbirth (odds ratio 1.8-5.8) 1
Important Clinical Caveats
The AFI has significant limitations that clinicians must understand:
- AFI correlates poorly with actual amniotic fluid volume, particularly in cases of oligohydramnios and polyhydramnios 4, 5
- Compared to the single deepest vertical pocket (SDVP) method, AFI diagnoses more cases of oligohydramnios and leads to more obstetric interventions without improving perinatal outcomes 4
- AFI should not be used to determine management in isolation but rather as part of overall fetal well-being assessment 4
Use in Fetal Surveillance
AFI is commonly incorporated into:
- Modified biophysical profile (mBPP): Combines AFI with non-stress test (NST) 1
- Full biophysical profile (BPP): Uses maximum vertical pocket rather than AFI 1
The volume of fluid corresponding to each centimeter of AFI changes throughout pregnancy, with approximately 61% more fluid per AFI centimeter at 30 weeks compared to 20 weeks 6. This means the same AFI value represents different actual fluid volumes at different gestational ages.