Amniotic Fluid Index Calculator

Interpret AFI measurements for oligohydramnios and polyhydramnios assessment. Enter values for instant results with step-by-step formulas.

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Formula

AFI = Q1 + Q2 + Q3 + Q4 (sum of deepest vertical pocket in each quadrant)

The uterus is divided into four quadrants using the umbilicus and linea nigra. The deepest vertical pocket of clear amniotic fluid (free of cord and fetal parts) is measured in centimeters in each quadrant. Normal AFI ranges from 5-24 cm. Oligohydramnios is defined as AFI < 5 cm, and polyhydramnios as AFI > 24 cm.

Worked Examples

Example 1: Normal AFI at 34 Weeks

Problem: A 34-week pregnancy with ultrasound quadrant measurements: Q1 = 4.5 cm, Q2 = 5.0 cm, Q3 = 4.0 cm, Q4 = 3.5 cm. SDP = 5.0 cm.

Solution: AFI = Q1 + Q2 + Q3 + Q4\nAFI = 4.5 + 5.0 + 4.0 + 3.5 = 17.0 cm\nNormal range at 34 weeks: 7.2 - 22.0 cm\n17.0 cm falls within the 50th-95th percentile range\nSDP = 5.0 cm (normal: 2-8 cm)

Result: AFI: 17.0 cm | Normal | 50th-95th percentile | No intervention needed

Example 2: Oligohydramnios at 38 Weeks

Problem: A 38-week pregnancy with quadrant measurements: Q1 = 1.2 cm, Q2 = 1.5 cm, Q3 = 0.8 cm, Q4 = 1.0 cm. SDP = 1.5 cm.

Solution: AFI = Q1 + Q2 + Q3 + Q4\nAFI = 1.2 + 1.5 + 0.8 + 1.0 = 4.5 cm\nNormal range at 38 weeks: 6.5 - 19.4 cm\n4.5 cm is below the 5th percentile\nSDP = 1.5 cm (below 2 cm threshold)

Result: AFI: 4.5 cm | Oligohydramnios | Below 5th percentile | Delivery consideration recommended

Frequently Asked Questions

What is the Amniotic Fluid Index and how is it measured?

The Amniotic Fluid Index (AFI) is a semi-quantitative method of assessing amniotic fluid volume during pregnancy using ultrasound. The technique involves dividing the uterus into four quadrants using the umbilicus and linea nigra as landmarks. The deepest vertical pocket of amniotic fluid free of fetal parts and umbilical cord is measured in each quadrant in centimeters. The four measurements are summed to produce the AFI. The examination is performed with the patient in the supine position and the ultrasound transducer held perpendicular to the floor. Normal AFI values range from approximately 5 to 24 centimeters, though expected values vary somewhat with gestational age.

How does amniotic fluid volume change throughout pregnancy?

Amniotic fluid volume follows a predictable pattern throughout pregnancy. It increases steadily from the first trimester, reaching approximately 200 mL by 16 weeks and 800 mL by 28 weeks. Fluid volume peaks at approximately 1000 mL around 34 to 36 weeks of gestation. After 36 weeks, the volume gradually decreases, averaging about 800 mL at 40 weeks and potentially dropping to 400 mL or less at 42 weeks. This physiological decline is reflected in the AFI reference ranges, which show lower values at term and post-term gestational ages. Understanding this normal progression is essential for correctly interpreting AFI measurements, as a value that is normal at 34 weeks might be borderline low at 40 weeks.

Can dehydration affect amniotic fluid levels?

Yes, maternal dehydration has been shown to decrease amniotic fluid volume, and conversely, hydration can temporarily increase it. Studies have demonstrated that oral hydration of 2 liters of water can increase AFI by an average of 3 to 4 centimeters within 2 to 4 hours. Intravenous hydration with isotonic fluids can produce even more pronounced effects. This relationship exists because approximately 500 to 1000 mL of amniotic fluid is exchanged daily, with maternal plasma volume and osmolality directly influencing fetal urine production. However, hydration therapy only treats the symptom rather than the underlying cause of oligohydramnios, and its effects are transient. Clinicians should ensure patients are well-hydrated before diagnosing oligohydramnios to avoid false positive results.

When should amnioinfusion be considered for low fluid levels?

Amnioinfusion is the instillation of fluid (typically normal saline) into the amniotic cavity through an intrauterine pressure catheter during labor or through amniocentesis antepartum. During labor, transcervical amnioinfusion is most commonly used to treat recurrent variable decelerations caused by cord compression from oligohydramnios, and studies show it can reduce the rate of cesarean delivery for fetal heart rate abnormalities. Antepartum transabdominal amnioinfusion has been used diagnostically to improve ultrasound visualization of fetal anatomy when oligohydramnios limits the examination, and therapeutically in previable pregnancies to prevent pulmonary hypoplasia. However, routine prophylactic amnioinfusion is not recommended, and the procedure carries risks including chorioamnionitis, cord prolapse, and placental abruption.

How accurate are the results from Amniotic Fluid Index Calculator?

All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.

How do I interpret the result?

Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.