Amniotic Fluid Index (AFI) Calculator — Oligohydramnios & Polyhydramnios

How to Use This Calculator

1. Enter the deepest vertical pocket depth (cm) from each of the four uterine quadrants.
2. AFI total and fluid status appear instantly.
3. Single Deepest Pocket tab: enter SDP measurement as an alternative assessment.
4. Clinical Interpretation tab: enter AFI and gestational age for causes and management guidance.
5. Professional tier: add GA, SDP, PROM status, and diabetes for comprehensive clinical plan.

Formula

Example

Frequently Asked Questions

• What is the Amniotic Fluid Index and how is it measured? ▼
  The Amniotic Fluid Index (AFI) is a standardized ultrasound measurement used to assess the volume of amniotic fluid surrounding the fetus. It was developed by Thomas Moore and colleagues and published in 1990. To measure the AFI, the uterus is mentally divided into four equal quadrants by a vertical line at the umbilicus and a horizontal line at the midline. Using a transducer held perpendicular to the floor (not angled), the examiner identifies the largest vertical pocket of amniotic fluid in each quadrant, free of fetal parts and umbilical cord. The depth of each pocket in centimeters is measured and the four values are summed: AFI = Q1 + Q2 + Q3 + Q4. The normal AFI range at term (37–42 weeks) is 8–25 cm. Amniotic fluid peaks around 32–34 weeks (mean approximately 14–15 cm) and gradually decreases toward term. AFI below 5 cm is classified as oligohydramnios; AFI 5–8 cm is considered low normal and warrants increased surveillance; AFI above 25 cm is classified as polyhydramnios. The Single Deepest Pocket (SDP) or Maximum Vertical Pocket (MVP) is an alternative method where only the largest single pocket anywhere in the uterus is measured — normal SDP range is 2–8 cm. Multiple studies, including the PORTO trial, suggest SDP is at least as accurate as AFI for predicting adverse perinatal outcomes with fewer unnecessary interventions.
• What causes oligohydramnios and how serious is it? ▼
  Oligohydramnios (AFI <5 cm or SDP <2 cm) is a significant finding associated with increased perinatal morbidity and mortality. Amniotic fluid is primarily produced by fetal urination after approximately 16 weeks of gestation; before 16 weeks, it is derived from transudation across fetal membranes. Therefore, any condition that reduces fetal urine production or increases fluid loss can cause oligohydramnios. The most common cause at term is prelabor rupture of membranes (PROM) — fluid leaks through the ruptured membranes and is not replenished at the same rate. Fetal causes include renal agenesis (bilateral — Potter sequence, incompatible with life), multicystic dysplastic kidneys, obstructive uropathies (posterior urethral valves in males — the most common surgically correctable cause), and any cause of severe fetal growth restriction where placental insufficiency reduces fetal blood flow to the kidneys. Maternal causes include significant dehydration, NSAIDs and COX-2 inhibitors (which constrict fetal renal vasculature — avoid after 32 weeks), ACE inhibitors and ARBs (fetotoxic — contraindicated in pregnancy). Post-term pregnancy is associated with declining amniotic fluid as placental function decreases. Severe oligohydramnios, particularly before 22 weeks (lethal oligohydramnios), causes pulmonary hypoplasia because normal lung development requires breathing movements against adequate fluid, and limb contractures from restricted movement.
• What causes polyhydramnios and when should it be investigated? ▼
  Polyhydramnios (AFI >25 cm or SDP >8 cm) affects approximately 1–2% of pregnancies and has identifiable causes in approximately 50% of cases; the remainder are idiopathic. Amniotic fluid volume reflects the balance between fetal urinary production and fetal swallowing — any condition increasing production or impairing swallowing causes excess fluid. Maternal causes: pre-gestational and gestational diabetes mellitus is the most common identifiable cause, accounting for approximately 25% of polyhydramnios. Fetal hyperglycemia and osmotic diuresis increase urine output. The degree of polyhydramnios correlates with glycemic control. Fetal causes: Gastrointestinal anomalies that impair swallowing are important — esophageal atresia (often associated with polyhydramnios ± absent stomach bubble), duodenal atresia (double bubble sign on ultrasound), intestinal atresias, and cleft palate. Central nervous system anomalies impair swallowing coordination (anencephaly, spina bifida). Fetal anemia from any cause (Rh alloimmunization, feto-maternal hemorrhage) increases cardiac output and urine production. Twin-to-twin transfusion syndrome (TTTS) in monochorionic twins causes polyhydramnios in the recipient twin and oligohydramnios in the donor twin — the hallmark ultrasound finding. All cases of polyhydramnios should be evaluated with detailed fetal anatomy scan, maternal glucose tolerance test, blood group antibody screen, and fetal medicine consultation if AFI exceeds 30 cm.
• What is the AFI at different gestational ages? ▼
  Amniotic fluid volume changes substantially across gestation, peaking in mid-pregnancy and declining in the third trimester. Understanding normal gestational age-specific ranges is essential for accurate interpretation of AFI measurements. In the first trimester (8–12 weeks), amniotic fluid is minimal and not measured by AFI. In the second trimester, AFI rises progressively: at 16 weeks the mean AFI is approximately 11–12 cm; at 20 weeks approximately 14 cm; at 24 weeks approximately 14–15 cm (peak range). In the third trimester, AFI remains relatively stable then declines: at 28 weeks mean approximately 14 cm; at 32 weeks approximately 14–15 cm (absolute peak); at 36 weeks approximately 12–13 cm; at 38–39 weeks approximately 11 cm; at 40 weeks approximately 10–11 cm; and at 41–42 weeks the mean declines to approximately 8–9 cm. Post-term decline is clinically significant — normally aging placentas produce decreasing fluid volumes, and oligohydramnios at 41+ weeks is an indication for induction. Reference values from Moore and Cayle (1990) and subsequent studies show the 5th percentile at 40 weeks is approximately 5 cm and the 95th percentile is approximately 22–24 cm. These ranges explain why identical AFI values have different significance at different gestational ages; dedicated gestational age-corrected nomograms should be used for precise percentile determination.
• What is the biophysical profile and how does AFI fit into it? ▼
  The Biophysical Profile (BPP) is a comprehensive fetal wellbeing assessment tool developed by Frank Manning in 1980. It combines ultrasound observation of four fetal behavioral parameters with the non-stress test (NST) for a maximum score of 10. Each component is scored 0 (absent/abnormal) or 2 (present/normal). The five BPP components are: Fetal Movement (≥3 gross body movements in 30 minutes = 2 points), Fetal Tone (≥1 episode of limb extension with return to flexion = 2 points), Fetal Breathing (≥1 episode of rhythmic breathing ≥30 seconds in 30 minutes = 2 points), Non-Stress Test (reactive NST = 2 points), and Amniotic Fluid Volume — which is the AFI component. For the BPP, amniotic fluid is scored as 2 points (normal) if at least one pocket measuring ≥2 cm in two perpendicular planes is present, or as 0 points (abnormal) if no such pocket exists. A BPP score of 8–10 is reassuring; 6 is equivocal and requires repeat testing within 24 hours; 4 or below is abnormal and usually warrants delivery or intensive evaluation. The Modified BPP uses only the NST plus AFI as a quick screening test — a reactive NST with normal AFI is reassuring; an abnormal NST or AFI triggers a full BPP or additional evaluation. AFI is thus a core component of standard antenatal fetal surveillance protocols and biophysical profile scoring.

Related Calculators