Amniotic Fluid Index Calculator
Calculate amniotic fluid index (AFI) from four-quadrant ultrasound measurements, compare with single deepest pocket (SDP) context, and review boundary-aware interpretation guidance for prenatal follow-up discussion.
Medical Disclaimer
This calculator is an educational interpretation aid. It does not diagnose fetal conditions, determine delivery timing, or replace clinician-guided obstetric care.
Calculate AFI
Your Results
Quadrant Breakdown
Formula Trace
AFI = Q1 + Q2 + Q3 + Q4
AFI = 4.5 + 4.0 + 3.5 + 4.0
AFI = 16.0 cm
Interpretation and Follow-up
Possible Cause Context
- Fluid pattern is commonly interpreted as reference-range context.
Associated Risk Context
- No specific excess risk is inferred from AFI alone in reference-range context.
Practical Recommendations
- Continue routine prenatal follow-up unless symptom profile changes.
- Interpret AFI together with fetal testing, symptoms, and full obstetric context.
- Use repeat standardized measurements to confirm trend before major decisions when stable.
Safety Reminder
AFI output is educational context only and must not replace obstetric diagnosis, triage, or treatment decisions.
Editorial & Review Information
Reviewed on: 2026-02-26
Published on: 2025-12-04
Author: LumoCalculator Editorial Team
Editorial review: AFI formula consistency, SDP boundary wording, urgency-language safety, source-link availability, and medical-scope statements were reviewed for C-phase alignment.
Purpose and scope: Supports educational review of amniotic-fluid measurement trends in prenatal care context. Not intended for emergency triage, diagnosis, or treatment decisions.
Use Scenarios
Scenario 1: Follow-up trend review
Compare AFI values across serial scans collected with the same protocol to see whether fluid context is stable, improving, or drifting toward threshold zones.
Scenario 2: Visit preparation
Use AFI plus SDP context as a structured summary before prenatal appointments to improve discussion quality about surveillance cadence and next-step testing.
Scenario 3: Quality-control check
Re-run calculations from documented quadrant values to verify reporting consistency and reduce manual summation mistakes in educational or audit workflows.
Formula Explanation
Core Structure
AFI and SDP are screening-context metrics, not standalone diagnosis outputs. AFI summarizes four quadrant pockets, while SDP highlights the largest single pocket. Some protocols rely more on one method than the other depending on gestational stage and care setting.
Measurement quality is strongly technique dependent. Probe angle, inclusion of cord loops, or inconsistent quadrant interpretation can shift values enough to alter category labels. For serial decision support, consistency of protocol often matters as much as single absolute numbers.
Clinical interpretation should integrate AFI with fetal movement, non-stress testing or biophysical profile context, growth trajectory, and symptom profile. AFI should not be used in isolation for treatment or delivery decisions.
AFI and SDP Reference Context
AFI Bands
SDP Bands
AFI by Gestational Stage
AFI trends upward through second trimester.
Common peak-volume period in late second to early third trimester.
Values may plateau then gradually decline near term.
Post-term decline can occur and needs clinical context.
Measurement Protocol and Technique Control
Protocol Steps
- Divide the uterus into four quadrants using maternal midline and umbilical reference.
- Hold the transducer perpendicular to the floor for each quadrant scan.
- Measure the deepest vertical fluid pocket in each quadrant, excluding cord and fetal parts.
- Record each pocket in centimeters and sum all four values for AFI.
- When available, use color Doppler support if cord inclusion is uncertain.
Technique Notes
- Use consistent positioning and measurement protocol for serial trend comparison.
- Avoid oblique measurements that overestimate or underestimate pocket depth.
- Interpret AFI with gestational age, fetal testing, and full obstetric context.
Small technique differences can move AFI by meaningful margins. For trend reliability, keep operator approach, patient position, and documentation style as consistent as possible.
Example Cases
Case 1: Reference-range context
Q1 4.5, Q2 4.0, Q3 3.5, Q4 4.0 gives AFI 16.0 cm. SDP is 4.5 cm. This is usually interpreted as reference-range context and may support routine follow-up if other assessments are stable.
Case 2: Low-fluid threshold context
Q1 1.2, Q2 1.0, Q3 1.1, Q4 0.9 gives AFI 4.2 cm with SDP 1.2 cm. This pattern falls below common low-fluid thresholds and typically requires prompt clinician review with broader fetal-surveillance data.
Case 3: High-fluid context
Q1 8.0, Q2 7.5, Q3 7.0, Q4 7.5 gives AFI 30.0 cm with SDP 8.0 cm. This suggests high-fluid context and usually warrants structured review for maternal and fetal contributors.
Common Mistakes and Practical Fixes
Mistake 1: Using oblique measurement angle
Fix: keep pocket measurement vertical and perpendicular to floor to reduce angle-induced bias.
Mistake 2: Including cord in pocket depth
Fix: confirm fluid-only pocket and use Doppler support when uncertainty exists.
Mistake 3: Overreacting to one scan
Fix: use trend plus fetal testing context before major decisions when the clinical picture is stable.
Mistake 4: Using AFI as a standalone diagnosis
Fix: integrate AFI with obstetric exam, symptom profile, and clinician-guided care pathway.
8-Week Follow-up Framework
Weeks 1-2: Baseline and protocol lock
Confirm measurement protocol, collect baseline AFI and SDP context, and record gestational stage, symptoms, and related fetal-testing status.
Weeks 3-6: Trend monitoring
Repeat scans under consistent conditions. Compare directional trend, not only absolute values, and review whether context remains stable, improves, or drifts toward action thresholds.
Weeks 7-8: Decision-support review
Summarize AFI/SDP trend, symptoms, and testing context for clinician discussion on surveillance cadence, additional evaluation needs, and delivery-timing strategy.
Boundary Conditions
- Designed for educational support, not definitive diagnosis or emergency triage.
- AFI interpretation depends on gestational stage and full obstetric context.
- Technique variability can materially affect values; serial consistency is critical.
- Threshold wording may vary by institution and clinical guideline version.
- Does not model all maternal-fetal conditions that influence fluid balance.
- When clinician advice differs from calculator output, clinician advice takes priority.
Sources & References
- NCBI Bookshelf (StatPearls) - Antenatal Fetal Surveillance - Overview of fetal-surveillance methods and the role of fluid assessment in prenatal monitoring.
- NCBI Bookshelf (StatPearls) - Oligohydramnios - Clinical context, causes, and follow-up considerations for low amniotic-fluid patterns.
- NCBI Bookshelf (StatPearls) - Polyhydramnios - Clinical context, causes, and management considerations for high amniotic-fluid patterns.
- AAFP - Antenatal Fetal Surveillance - Primary-care review of fetal-surveillance frameworks including fluid assessment context.
- AAFP - Fetal Growth Restriction Before and After Birth - Context for growth-restriction evaluation where fluid assessment may influence monitoring strategy.
- Cleveland Clinic - Oligohydramnios - Patient-facing explanation of low-fluid causes and clinical follow-up context.
- Cleveland Clinic - Polyhydramnios - Patient-facing explanation of high-fluid causes, risks, and monitoring context.