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Obstetric Imaging
Demonstration data

Amniotic Fluid Index (AFI)

Categorises amniotic fluid volume from the four-quadrant amniotic fluid index to identify oligohydramnios or polyhydramnios.

The amniotic fluid index is the sum of the deepest vertical, cord- and part-free fluid pocket measured in each of the four uterine quadrants. It is a semi-quantitative estimate of amniotic fluid volume in the second and third trimesters and complements the single deepest pocket technique.

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📐 Formula

AFI = sum of the deepest vertical fluid pocket in each of the four uterine quadrants

When to Use

  • Third-trimester assessment of fetal wellbeing and as part of the biophysical profile.
  • Evaluation of suspected oligohydramnios (growth restriction, ruptured membranes) or polyhydramnios (diabetes, fetal anomaly).
  • Serial monitoring of at-risk pregnancies.

How It Works

  • Divide the uterus into four quadrants using the umbilicus and linea nigra.
  • Measure the deepest vertical fluid pocket in each quadrant, avoiding cord and fetal parts.
  • Sum the four values to obtain the AFI in cm.

Interpreting the Result

  • AFI <5 cm: oligohydramnios.
  • AFI 5–8 cm: low-normal (borderline).
  • AFI 8–18 cm: normal.
  • AFI 18–24 cm: high-normal.
  • AFI >24–25 cm: polyhydramnios.

Limitations & Pitfalls

  • AFI overestimates low fluid states and diagnoses oligohydramnios more often than the single deepest pocket, without improving outcomes — the single deepest pocket is preferred by many guidelines.
  • Values are gestational-age dependent; term thresholds do not apply to the early second trimester.
  • Operator technique and fetal position affect reproducibility.

References

  • Phelan JP, Smith CV, Broussard P, Small M. Amniotic fluid volume assessment with the four-quadrant technique at 36-42 weeks gestation. J Reprod Med. 1987;32(7):540-542.
  • Moore TR, Cayle JE. The amniotic fluid index in normal human pregnancy. Am J Obstet Gynecol. 1990;162(5):1168-1173.

⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.