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Amniotic Fluid Index (AFI) Calculator

Q: What is a normal AFI?

Roughly 5–24 cm in the third trimester, with typical values of 10–20 cm. AFI varies with gestational age, peaking around 33–34 weeks and falling gently towards term.

Q: What causes low or high amniotic fluid?

Low fluid can follow ruptured membranes, placental insufficiency, post-term pregnancy or some fetal kidney conditions. High fluid is linked to diabetes, twin pregnancy, some fetal swallowing problems or infections, and is often unexplained. Any abnormal value needs review by your care team.

Q: Is AFI or single deepest pocket better?

Both are accepted. Studies show that using the single deepest pocket to diagnose oligohydramnios leads to fewer interventions without worse outcomes, so SMFM recommends it for that purpose.

Add the four quadrant pocket depths to get the amniotic fluid index (AFI), plus the single deepest pocket, and see whether the values fall in the oligohydramnios, normal or polyhydramnios range.

Right upper quadrant pocket

Left upper quadrant pocket

Right lower quadrant pocket

Left lower quadrant pocket

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How it works

AFI (cm) = sum of the deepest vertical pocket in each of the four uterine quadrants · single deepest pocket = largest of the four · AFI ≤ 5 cm low, ≥ 24 cm high; SDP < 2 cm low, ≥ 8 cm high

The amniotic fluid index (Phelan et al., 1987) divides the uterus into four quadrants around the navel and adds the deepest vertical pocket of fluid free of cord and limbs in each, measured in centimetres. ACOG and the Society for Maternal-Fetal Medicine define oligohydramnios as an AFI of 5 cm or less (or a single deepest pocket under 2 cm) and polyhydramnios as an AFI of 24 cm or more (or a pocket of 8 cm or more). SMFM now prefers the single deepest pocket for diagnosing low fluid, because AFI labels more pregnancies as abnormal without improving outcomes.

Worked example

Pockets of 4.2, 3.8, 3.5 and 3.0 cm give AFI = 4.2 + 3.8 + 3.5 + 3.0 = 14.5 cm, within the normal range (5.1–23.9 cm). The single deepest pocket is 4.2 cm, also normal.

Sources: ACOG Practice Bulletin / SMFM — Amniotic fluid assessment (SMFM Consult Series #46, 2018) · Phelan JP et al. (1987) Amniotic fluid volume assessment with the four-quadrant technique at 36–42 weeks. J Reprod Med 32:540–542

Frequently asked questions

What is a normal AFI?

Roughly 5–24 cm in the third trimester, with typical values of 10–20 cm. AFI varies with gestational age, peaking around 33–34 weeks and falling gently towards term.

What causes low or high amniotic fluid?

Low fluid can follow ruptured membranes, placental insufficiency, post-term pregnancy or some fetal kidney conditions. High fluid is linked to diabetes, twin pregnancy, some fetal swallowing problems or infections, and is often unexplained. Any abnormal value needs review by your care team.

Is AFI or single deepest pocket better?

Both are accepted. Studies show that using the single deepest pocket to diagnose oligohydramnios leads to fewer interventions without worse outcomes, so SMFM recommends it for that purpose.

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This calculator is for education only and does not replace your obstetric care team. Ultrasound measurements, their interpretation and any dating or diagnosis must come from a qualified sonographer or clinician.