Amniotic Fluid Index Calculator
Enter the deepest vertical amniotic fluid pocket measured in each of the four quadrants of the uterus, and this calculator returns your Amniotic Fluid Index (AFI) in cm along with its clinical classification.
Calculate AFI from Four Quadrant Measurements
Amniotic Fluid Index Formula
Where:
- \(Q_1\) — largest vertical fluid pocket in the upper right quadrant (cm)
- \(Q_2\) — largest vertical fluid pocket in the upper left quadrant (cm)
- \(Q_3\) — largest vertical fluid pocket in the lower right quadrant (cm)
- \(Q_4\) — largest vertical fluid pocket in the lower left quadrant (cm)
The amniotic fluid index is simply the sum of the four deepest vertical pockets, one measured in each quadrant of the uterus. It quantifies the total amount of amniotic fluid surrounding the fetus on ultrasound.
Amniotic Fluid Index – Calculation Example
Example: an ultrasound at 34 weeks measures the following deepest vertical pockets: 4.2 cm in the upper right quadrant, 3.8 cm in the upper left, 5.1 cm in the lower right and 4.5 cm in the lower left.
\[ AFI = 4.2 + 3.8 + 5.1 + 4.5 = 17.6 \text{ cm} \]
Result: AFI = 17.6 cm, which falls within the normal range (8–24 cm). The lower right pocket contributes the largest share of the total (about 29%), while the other quadrants each contribute roughly 21–26%.
How to use the calculator
- Perform or obtain an ultrasound scan and identify the four quadrants of the uterus, divided by the umbilicus (navel) and the linea nigra as vertical and horizontal reference lines.
- In each quadrant, measure the largest vertical pocket of amniotic fluid that is free of fetal limbs and the umbilical cord, in centimeters.
- Enter each value into the matching field: Upper right quadrant, Upper left quadrant, Lower right quadrant and Lower left quadrant.
- Read the result: the total Amniotic fluid index (AFI) in cm, its Classification (normal, borderline low, oligohydramnios or polyhydramnios), and a per-quadrant breakdown showing each pocket's share of the total.
What is the amniotic fluid index?
The amniotic fluid index (AFI) is a semi-quantitative ultrasound measure of the amount of amniotic fluid surrounding the fetus. It was described by Phelan and colleagues in 1987 and remains one of the two standard methods — along with the single deepest pocket (SDP) — used to assess fluid volume during prenatal monitoring, typically from about 20 weeks of gestation onward.
Amniotic fluid plays several vital roles: it cushions the fetus, allows normal movement and lung development, maintains a stable temperature, and permits umbilical cord movement without compression. The fluid is constantly renewed — the fetus swallows it and excretes it as urine — so the AFI reflects a dynamic balance. Too little fluid (oligohydramnios) can restrict lung growth and compress the cord; too much fluid (polyhydramnios) can signal gestational diabetes, fetal differences, or increase the risk of preterm labor and malpresentation.
The standard interpretation thresholds are: AFI below 5 cm — oligohydramnios; 5 to 8 cm — borderline low; 8 to 24 cm — normal; above 24 cm — polyhydramnios. These ranges apply to most pregnancies after about 20 weeks, but exact cut-offs can vary slightly by gestational age, by clinic protocol and by whether the SDP method is used instead. Always discuss the result with your obstetric provider, who will interpret it in the context of your full clinical picture — a single AFI value is never a diagnosis on its own.
Assumptions, limitations and practical tips
- Each pocket should be a true vertical measurement. Measure the deepest pocket in each quadrant with the ultrasound probe held vertically (in the transverse plane), excluding pockets that contain cord or limbs.
- Values above 8 cm in a single quadrant are unusual when measuring for AFI. With the single deepest pocket method, however, a pocket over 8 cm is itself one criterion for polyhydramnios.
- Gestational age matters. Normal fluid volume changes across pregnancy; if you are unsure of your dates, a gestational age calculator or an early ultrasound (see the crown rump length calculator) can help establish them.
- AFI is operator-dependent. Inter-observer differences of 1–2 cm are common, so a borderline result is often repeated rather than acted on immediately.
- This calculator is informational, not diagnostic. An abnormal or borderline AFI should always be reviewed by your healthcare provider, who may order repeat scans, additional testing or closer monitoring. If you have polyhydramnios related to elevated blood sugar, the gestational diabetes diet guide may also be helpful.
- In the third trimester, daily fetal movement awareness remains an important safety check alongside fluid monitoring — the baby kick count calculator can help you track it.
Frequently Asked Questions
What is a normal amniotic fluid index by gestational age?
For most singleton pregnancies after about 20 weeks, an AFI between 8 and 24 cm is considered normal. Fluid volume naturally peaks around 30–34 weeks and gradually declines toward term, so an AFI of 7 cm at 40 weeks may be less concerning than the same value at 28 weeks. Many clinics use slightly different cut-offs (some use 5–25 cm). Always have your provider interpret the number alongside your gestational age and overall clinical picture.
How is the AFI measured on ultrasound?
The examiner divides the maternal abdomen into four quadrants using the midline (linea nigra) and a horizontal line through the navel as guides. With the probe held vertically in the transverse plane, the deepest amniotic fluid pocket in each quadrant is measured — the pocket must be free of fetal limbs and umbilical cord. The four measurements in centimeters are then added together. The whole scan usually takes only a few minutes and involves no radiation.
What is the difference between AFI and the single deepest pocket (SDP)?
AFI sums the deepest pockets from all four quadrants; SDP (also called maximum vertical pocket, MVP) uses only the single largest pocket anywhere in the uterus. Typical SDP thresholds are: below 2 cm suggests oligohydramnios, above 8 cm supports polyhydramnios. Some guidelines, especially in twin pregnancies or later gestations, prefer SDP because AFI tends to overdiagnose low fluid. The two methods are not interchangeable, so note which one your report uses.
Can I calculate AFI from measurements in millimeters or inches?
Yes, but convert to centimeters first. Divide millimeters by 10 (e.g., 42 mm = 4.2 cm) or multiply inches by 2.54 (e.g., 2 in ≈ 5.1 cm). Ultrasound reports normally state pocket depths in centimeters, so conversion is rarely needed in practice. Mixing units is a common source of error — an AFI of 176 would be nonsense unless the inputs were accidentally entered in millimeters.
What causes a low AFI (oligohydramnios) and can it improve?
Common causes include ruptured membranes (leaking fluid), placental insufficiency, post-term pregnancy, certain fetal kidney or urinary tract issues, and some medications. Mild borderline-low readings are often just measurement variation and may normalize on a repeat scan — differences of 1–2 cm between examiners are common. Maternal hydration can modestly affect fluid volume. Low AFI in later pregnancy warrants prompt medical review, so contact your provider rather than self-managing.
What are common mistakes when computing or interpreting AFI?
Frequent errors include measuring a pocket that contains cord or fetal limbs (inflating the value), measuring in the wrong plane (oblique rather than true vertical), summing pockets from the same quadrant instead of one per quadrant, and confusing AFI with SDP thresholds. Also remember that a single reading is not diagnostic: borderline values are usually repeated before any decision is made, and repeat scans are ideally done by the same operator or unit for consistency.
Does a high AFI always mean something is wrong with the baby?
No. Mild polyhydramnios (AFI just above 24 cm) is often idiopathic — no cause is found — and many such pregnancies proceed normally. More significant elevations are associated with gestational diabetes, fetal swallowing differences, or, less commonly, structural or chromosomal conditions. A high reading typically triggers a glucose screening test, a detailed ultrasound, and follow-up fluid checks. Discuss the result with your obstetric provider to determine which steps apply to you.