Amniotic Fluid Index Calculator

The Amniotic Fluid Index (AFI) is a standardized method for assessing amniotic fluid volume during pregnancy using ultrasound. MEASUREMENT METHOD: The uterus is divided into 4 quadrants using the maternal umbilicus and linea nigra as reference points. In each quadrant, the deepest vertical pocket of amniotic fluid (free of umbilical cord and fetal parts) is measured perpendicular to the floor. The four measurements are summed to get the AFI. FORMULA: AFI = Q1 + Q2 + Q3 + Q4 (all in cm). HOW TO PERFORM: 1. Patient lies supine (slight left tilt acceptable). 2. Transducer held perpendicular to floor. 3. Identify deepest pocket in each quadrant. 4. Pocket must be free of cord and fetal extremities. 5. Measure vertical dimension only. 6. Sum all four quadrants. NORMAL VALUES: Normal AFI: 8-24 cm (some sources use 5-25 cm). Average at term: ~12-14 cm. Peak volume: Around 32-34 weeks gestation. WHY IT MATTERS: Too little (oligohydramnios): May indicate fetal distress, renal problems, or membrane rupture. Too much (polyhydramnios): May indicate maternal diabetes, fetal anomalies, or swallowing issues. LIMITATIONS: Operator-dependent technique. Inter-observer variability exists. Single measurement is a snapshot in time. Should be interpreted with clinical context.

Both AFI and Single Deepest Pocket (SDP) are ultrasound methods to assess amniotic fluid, but they differ in approach and application. AFI (AMNIOTIC FLUID INDEX): Measures 4 quadrants and sums them. Normal: 8-24 cm. More comprehensive assessment. Traditional standard for third trimester. May overdiagnose oligohydramnios. SDP (SINGLE DEEPEST POCKET): Measures only the deepest pocket in the entire uterus. Normal: 2-8 cm. Simpler, faster measurement. Often preferred in first/second trimester. May be more appropriate for multiple gestations. CUTOFFS FOR ABNORMALITY: | Condition | AFI | SDP |. | Oligohydramnios | <5 cm | <2 cm |. | Normal | 5-24 cm | 2-8 cm |. | Polyhydramnios | >24 cm | >8 cm |. WHICH TO USE: AFI: Standard for singleton pregnancies in third trimester. SDP: Preferred for twins/multiples (each sac measured separately). SDP: May be preferred when AFI shows borderline low values. Either: Both have predictive value for outcomes. EVIDENCE: Studies suggest SDP may lead to fewer interventions without worse outcomes. AFI may have higher false-positive rate for oligohydramnios. Current practice often uses both for clinical decision-making. ACOG notes both are acceptable methods.

Oligohydramnios (AFI <5 cm or SDP <2 cm) has multiple potential causes that vary by gestational age. COMMON CAUSES: 1. RUPTURE OF MEMBRANES (MOST COMMON). PROM: Premature rupture at term. PPROM: Preterm premature rupture. May be obvious or occult (slow leak). Tests: Nitrazine, ferning, AmniSure, ultrasound. 2. UTEROPLACENTAL INSUFFICIENCY. Placenta not functioning optimally. Often associated with IUGR. Can indicate fetal hypoxia. Evaluate with Doppler studies. 3. FETAL RENAL ANOMALIES. Bilateral renal agenesis (Potter syndrome). Bilateral obstruction. Polycystic kidney disease. Causes anhydramnios or severe oligohydramnios. 4. FETAL GROWTH RESTRICTION (IUGR). Reduced fetal urine output. Blood redistributed to vital organs. Poor placental function. 5. POST-TERM PREGNANCY. Placental function declines. Natural decrease in AFI after 40 weeks. Closer monitoring needed. 6. MATERNAL FACTORS. Dehydration (reversible with hydration). Hypertension/preeclampsia. Medications (ACE inhibitors, NSAIDs). Maternal chronic disease. 7. CHROMOSOMAL ABNORMALITIES. Associated with various aneuploidies. Often with other findings. 8. TWIN-TO-TWIN TRANSFUSION. Donor twin: oligohydramnios. Recipient twin: polyhydramnios. In monochorionic twins. CONSEQUENCES OF OLIGOHYDRAMNIOS: Cord compression (variable decelerations). Pulmonary hypoplasia (if prolonged/severe). Limb deformities (Potter sequence). Higher cesarean rates. Meconium-stained fluid. EVALUATION STEPS: 1. Rule out ruptured membranes. 2. Assess fetal anatomy (especially kidneys/bladder). 3. Evaluate fetal growth. 4. Check umbilical artery Doppler. 5. Consider maternal hydration trial. 6. Increase fetal surveillance.

Polyhydramnios (AFI >24 cm or SDP >8 cm) occurs when amniotic fluid accumulates excessively. CAUSES BY CATEGORY: 1. IDIOPATHIC (50-60% of cases). No identifiable cause found. Usually mild polyhydramnios. Generally good prognosis. 2. MATERNAL DIABETES. Gestational or preexisting diabetes. Most common identifiable cause. Fetal hyperglycemia → polyuria. Screen all patients with polyhydramnios. 3. FETAL ANOMALIES (10-20%). GI OBSTRUCTION: Esophageal atresia. Duodenal atresia (double bubble sign). Intestinal atresia. Impaired swallowing. NEURAL TUBE DEFECTS: Anencephaly. Open spina bifida. THORACIC MASSES: CCAM (lung malformation). Diaphragmatic hernia. NEUROMUSCULAR DISORDERS: Conditions affecting swallowing. Myotonic dystrophy. 4. FETAL ANEMIA. Rh isoimmunization. Parvovirus B19 infection. Alpha-thalassemia. Fetal-maternal hemorrhage. 5. TWIN-TO-TWIN TRANSFUSION SYNDROME. Recipient twin: polyhydramnios. In monochorionic pregnancies. Requires specialist management. 6. FETAL INFECTIONS. CMV. Toxoplasmosis. Syphilis. 7. CHROMOSOMAL ABNORMALITIES. Trisomy 18, 21. Other aneuploidies. SEVERITY CLASSIFICATION: Mild: AFI 25-30 cm. Moderate: AFI 30-35 cm. Severe: AFI >35 cm. RISKS OF POLYHYDRAMNIOS: Preterm labor and delivery. Premature rupture of membranes. Cord prolapse (if membranes rupture). Malpresentation. Placental abruption. Postpartum hemorrhage (overdistended uterus). Maternal respiratory compromise (severe cases). WORKUP: 1. Glucose screening (GCT or OGTT). 2. Detailed fetal anatomy ultrasound. 3. Fetal echocardiogram if indicated. 4. Consider amniocentesis for karyotype and infection. 5. Serial growth scans. 6. Monitor for preterm labor.

Amniotic fluid volume follows a predictable pattern throughout pregnancy, understanding this helps interpret AFI values. VOLUME CHANGES BY TRIMESTER: FIRST TRIMESTER (Weeks 1-13): Very small volumes initially. Primarily from maternal plasma transudate. By 10 weeks: ~30 mL. By 12 weeks: ~60 mL. SECOND TRIMESTER (Weeks 14-27): Rapid increase in volume. Fetal urine production begins ~10-11 weeks. Fetal urine becomes main source. By 16 weeks: ~175 mL. By 20 weeks: ~400 mL. THIRD TRIMESTER (Weeks 28-40): Peaks around 32-34 weeks: ~800-1000 mL. Gradual decline toward term. At 40 weeks: ~600-800 mL. Post-term: Continues to decline. AFI BY GESTATIONAL AGE: 16-20 weeks: ~8-14 cm. 20-28 weeks: ~10-18 cm. 28-34 weeks: ~12-20 cm (peak). 34-40 weeks: ~10-20 cm. 40+ weeks: ~8-15 cm, declining. SOURCES OF AMNIOTIC FLUID: PRODUCTION: Fetal urine (main source after 20 weeks). Fetal lung secretions. Transudation across fetal skin (early pregnancy). Umbilical cord and placenta. REMOVAL/ABSORPTION: Fetal swallowing (main route). Intramembranous absorption. Flow across placenta. TURNOVER: Complete fluid exchange every 3 hours at term. Daily production at term: ~500-1200 mL. Fetal swallows ~500-1000 mL/day. WHY AFI DECREASES POST-TERM: Placental function declines. Fetal renal blood flow may decrease. Normal physiological change. Increased monitoring indicated. CLINICAL IMPLICATIONS: Know expected range for gestational age. A "normal" AFI at 40 weeks would be low at 32 weeks. Serial measurements track trends. Single measurement is a snapshot.

Both oligohydramnios and polyhydramnios carry risks that depend on severity, duration, and underlying cause. RISKS OF OLIGOHYDRAMNIOS: 1. CORD COMPRESSION. Less fluid cushion for cord. Variable decelerations on monitoring. Higher cesarean delivery rates. More common in labor. 2. PULMONARY HYPOPLASIA. Lungs need fluid for development. Risk increases with: Severity (anhydramnios worst), Duration (prolonged exposure), Early onset (<20-22 weeks). Can be lethal if severe. 3. MUSCULOSKELETAL DEFORMITIES. Potter sequence/facies. Limb positioning abnormalities. Clubfoot. Joint contractures. Usually from severe, prolonged cases. 4. INTRAUTERINE DEMISE. From cord compression. From underlying condition. Requires close monitoring. 5. MECONIUM ASPIRATION. More concentrated meconium. Higher aspiration risk. Worse outcomes if aspirated. 6. LOW APGAR SCORES. Related to delivery complications. Fetal stress before delivery. RISKS OF POLYHYDRAMNIOS: 1. PRETERM DELIVERY. From uterine overdistension. Spontaneous preterm labor. May rupture membranes early. Risk correlates with severity. 2. CORD PROLAPSE. Especially at membrane rupture. More space for cord to precede baby. Emergency situation. 3. MALPRESENTATION. Excessive room for movement. More breech, transverse lies. May complicate delivery. 4. PLACENTAL ABRUPTION. Sudden decompression at rupture. Can cause abruption. Monitor closely. 5. POSTPARTUM HEMORRHAGE. Overdistended uterus contracts poorly. Higher blood loss at delivery. Prepare for intervention. 6. UNDERLYING CONDITION RISKS. Many causes have their own risks. Diabetes affects both mom and baby. Anomalies may have independent prognosis. SEVERITY MATTERS: Mild abnormalities: Often good outcomes. Moderate: Increased surveillance needed. Severe: Significantly higher complication rates. Duration and cause also influence prognosis.

Accurate AFI measurement requires proper technique to ensure reliable and reproducible results. PREPARATION: Patient position: Supine (or slight left lateral tilt if hypotensive). Full bladder: Usually not necessary (may distort measurement). Equipment: Standard obstetric ultrasound with curved array probe. STEP-BY-STEP TECHNIQUE: 1. DIVIDE THE UTERUS INTO QUADRANTS. Use umbilicus as horizontal reference. Use linea nigra (midline) as vertical reference. This creates 4 quadrants: Upper right (Q1), Upper left (Q2), Lower left (Q3), Lower right (Q4). 2. POSITION THE TRANSDUCER. Hold perpendicular to the FLOOR (not to skin). Do not angle the transducer. This ensures true vertical measurement. 3. IDENTIFY POCKETS IN EACH QUADRANT. Look for largest pocket in each quadrant. Pocket must be: Free of umbilical cord, Free of fetal parts, At least 1 cm wide horizontally. 4. MEASURE CORRECTLY. Measure the vertical depth (perpendicular to floor). Measure the deepest part of each qualifying pocket. Record in centimeters. 5. SUM THE MEASUREMENTS. AFI = Q1 + Q2 + Q3 + Q4. Report as total in cm. COMMON ERRORS TO AVOID: ANGLING THE TRANSDUCER: Gives falsely high or low readings. Must be perpendicular to floor. INCLUDING CORD IN POCKET: Will falsely elevate reading. Look carefully with color Doppler if needed. INCLUDING FETAL PARTS: Pocket must be clear of extremities. NOT FINDING TRUE DEEPEST POCKET: Scan thoroughly. May miss a deeper pocket. MEASURING OBLIQUE RATHER THAN VERTICAL: Measure straight up and down. INTER-OBSERVER VARIABILITY: Studies show 1-2 cm variation between sonographers. Use consistent technique. Same operator for serial measurements if possible. TIPS FOR ACCURACY: Use high-resolution imaging. Take your time in each quadrant. Color Doppler helps identify cord. Practice standardized technique. Document measurement locations.

Management of abnormal AFI depends on the severity, underlying cause, and gestational age. Follow-up varies accordingly. OLIGOHYDRAMNIOS FOLLOW-UP: BORDERLINE LOW (AFI 5-8 cm): Repeat AFI in 1-2 weeks. Encourage maternal hydration. Review medications (stop NSAIDs if applicable). Fetal growth assessment if not recent. May normalize with hydration. MODERATE (AFI <5 cm): More frequent monitoring (weekly or twice weekly). NST (Non-Stress Test) 1-2x per week. Consider BPP (Biophysical Profile). Evaluate for rupture of membranes. Detailed anatomy if not done. Umbilical artery Doppler. Plan for possible early delivery. SEVERE (AFI <3 cm or anhydramnios): Immediate obstetric consultation. Hospitalization may be needed. Daily or twice daily NST. Evaluate for delivery depending on GA. Amnioinfusion during labor if needed. Pediatric consultation for pulmonary hypoplasia risk. POLYHYDRAMNIOS FOLLOW-UP: MILD (AFI 25-30 cm): Glucose testing if not done. Detailed anatomy scan if not done. Repeat AFI in 2-3 weeks. Usually benign, often idiopathic. MODERATE (AFI 30-35 cm): More detailed workup: Fetal anatomy survey, Fetal echo if indicated, Consider amniocentesis (karyotype, infection). Serial AFIs (every 1-2 weeks). Monitor for preterm labor. SEVERE (AFI >35 cm): Maternal-Fetal Medicine consultation. Full workup for cause. Frequent monitoring. Consider therapeutic amniocentesis (amnioreduction). Antenatal corticosteroids if <34 weeks. May need early delivery. Hospital admission if symptomatic (breathing difficulty). GENERAL PRINCIPLES: Identify and treat underlying cause when possible. Gestational age guides delivery timing. Serial measurements show trend. Multidisciplinary care for complex cases. Patient education about warning signs.