Obstetric Imaging
Live dataAmniotic 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.
📐 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.