What is the normal range for Amniotic Fluid Index (AFI)?

Normal Amniotic Fluid Index (AFI) Range

The normal range for Amniotic Fluid Index (AFI) is 5-24 cm, with values below 5 cm indicating oligohydramnios and values above 24 cm or >95th percentile for gestational age indicating polyhydramnios. 1

Understanding AFI Measurement

• 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 1
• AFI is an essential component of the modified Biophysical Profile (mBPP) for fetal surveillance 1

Normal AFI Values by Gestational Age

• AFI values vary throughout pregnancy, with maximal values occurring around 26 weeks of gestation 2
• 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 1
• Before 37 weeks, an AFI of 8 cm or greater is commonly used to define normal amniotic fluid 1

Clinical Significance of AFI Values

• Oligohydramnios (AFI ≤ 5 cm) is associated with:

  • Increased risk of stillbirth (odds ratio 2.6,95% CI 2.1-3.2) 1
  • Higher rates of intrapartum fetal heart rate abnormalities 1
  • Increased risk of cesarean delivery for fetal distress 3

• 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) 1
  • Independent association with stillbirth (odds ratio 1.8,95% CI 1.4-2.2) 1

Volume Considerations

• The actual volume of amniotic fluid per centimeter of AFI changes during gestation 4
• At 20 weeks, each centimeter of AFI corresponds to approximately 25 mL of fluid 4
• At 30 weeks, each centimeter of AFI corresponds to approximately 41 mL of fluid 4
• At 34 weeks, each centimeter of AFI corresponds to approximately 35 mL of fluid 4

AFI vs. Maximum Vertical Pocket (MVP)

• Contemporary studies suggest that the Maximum Vertical Pocket (MVP) may perform better than AFI for clinical decision making 1
• 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 1
• Neither AFI nor MVP performs better than the other at preventing intrauterine fetal demise 1

Monitoring Recommendations

• For patients with normal AFI (> 8 cm) at less than 41 weeks gestation, weekly assessments are likely adequate 5
• For patients with low-normal AFI (5-8 cm) at less than 41 weeks gestation, twice-weekly assessments are recommended due to a higher risk (12.3%) of developing oligohydramnios within 4 days 5
• For all patients at or beyond 41 weeks gestation, twice-weekly AFI assessments are recommended regardless of the initial measurement 5

Racial/Ethnic Considerations

• Statistically significant differences in AFI have been observed among different racial/ethnic groups, particularly after 35 weeks gestation 2
• These differences are most prominent at the extremes of distribution (3rd and 97th percentiles) 2
• The absolute differences appear small and may not be clinically significant 2