Amniotic Fluid Index Measurement and Clinical Significance

Understanding AFI Measurement

The American College of Radiology defines the normal range for Amniotic Fluid Index (AFI) as 5-24 cm, with values below 5 cm indicating oligohydramnios and values above 24 cm or >95th percentile for gestational age indicating polyhydramnios 1, 2, 3
AFI is measured by summing 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 1
Normal amniotic fluid is reassuring, while both oligohydramnios and polyhydramnios are associated with adverse perinatal outcomes 4
AFI is an essential component of the modified Biophysical Profile (mBPP) for fetal surveillance 5

The definition of normal AFI varies with gestational age, though 5-24 cm is generally accepted across gestation 1
At term (37 weeks or beyond), an AFI of 5 cm or greater is typically considered adequate 6
Before 37 weeks, an AFI of 8 cm or greater is commonly used to define normal amniotic fluid 1

Oligohydramnios (AFI ≤ 5 cm) is associated with increased risk of stillbirth (odds ratio 2.6, 95% CI 2.1-3.2) and higher rates of intrapartum fetal heart rate abnormalities 7
Polyhydramnios (AFI > 24 cm or >95th percentile) is associated with increased risk of perinatal mortality (odds ratio 5.8, 95% CI 3.68-9.11) and independent association with stillbirth (odds ratio 1.8, 95% CI 1.4-2.2) 7

Contemporary studies suggest that the Maximum Vertical Pocket (MVP) may perform better than AFI for clinical decision making 7
Normal MVP is defined as ≥ 2 cm throughout gestation 1
More patients are diagnosed with oligohydramnios using AFI than MVP, resulting in more obstetric interventions 7
Neither AFI nor MVP performs better than the other at preventing intrauterine fetal demise 7