Late Pregnancy · Fluid Volume
Amniotic Fluid Index (AFI / SDP)
How amniotic fluid is measured in pregnancy ultrasound: AFI vs SDP, what normal looks like, oligohydramnios (low) and polyhydramnios (high) causes + management. NICE / SMFM.
Last reviewed 2 June 2026
Amniotic fluid — AFI or SDP
Amniotic Fluid Index / Single Deepest Pocket
Method
cm
wk
Enter the measurement and gestational age to interpret.
Educational tool only — not medical advice. SDP ≤ 2 cm is the SMFM and ACOG-preferred measure for oligohydramnios in routine surveillance (fewer false positives than AFI ≤ 5 cm — Nabhan 2008 Cochrane). For polyhydramnios, AFI is still commonly used. Amniotic fluid trends matter as much as a single value.
What does this mean?
Amniotic fluid is mostly fetal urine after about 20 weeks, so fluid volume is a real-time indicator of fetal kidney function, placental perfusion, and membrane integrity. Oligohydramnios (too little) is a red flag for membrane rupture, placental insufficiency / IUGR, post-term pregnancy, or congenital renal/urinary anomalies — at term, most centres recommend induction. Polyhydramnios (too much) commonly reflects maternal diabetes, multiple gestation, or fetal swallowing/ neuromuscular problems; severe cases raise the risk of preterm labour, malpresentation, and PPROM. The Cochrane meta- analysis (Nabhan 2008) found that SDP picks up the same poor outcomes as AFI while reducing unnecessary interventions, so SMFM/ACOG/ISUOG now prefer SDP ≤ 2 cm for oligohydramnios surveillance. Trends over successive scans matter as much as any single value.
What is amniotic fluid?
The fluid around baby in the womb. Mainly baby’s urine (after 16 wk) + lung secretions; recycled by baby swallowing + placenta. Cushions, helps muscle/lung development, protects cord.
Volume peaks ~34-36 weeks then gradually declines.
How it’s measured
- AFI: sum of deepest pocket in each of 4 quadrants. Normal 5-25 cm.
- SDP (single deepest pocket): largest vertical pocket without cord/limbs. Normal ≥2 cm.
SDP now preferred (NICE / SMFM) — fewer false-positive interventions.
Oligohydramnios (low fluid)
AFI ≤5 cm OR SDP <2 cm. Causes:
- Placental insufficiency (commonest late).
- ROM (waters broken / leaking).
- Fetal renal problem.
- Post-dates.
- Maternal dehydration.
- ACE inhibitors, NSAIDs.
Polyhydramnios (high fluid)
AFI ≥25 cm OR SDP >8 cm. Causes:
- Gestational diabetes (commonest).
- Fetal anomalies (gut atresia, neurological).
- Fetal anaemia.
- Twin-twin transfusion.
- Infection.
- Idiopathic.
Risks: preterm labour, malpresentation, cord prolapse, PPH.
Can drinking water help?
Mild dehydration correction can modestly improve AFI. Not a substitute for evaluation. 2L water/day for several days improved AFI in some studies. Tell midwife if trying.
Waters break (PROM / PPROM)
- PROM: at term, before labour starts.
- PPROM: before 37 weeks.
- Hospital assessment same day if suspected.
- Sterile speculum + nitrazine / ferning / AmniSure tests.
- PROM at term: induce within 12-24h usually.
- PPROM 24-34 wk: antibiotics + steroids + watchful waiting.
Fluid colour at ROM
- Clear / pale pink: normal.
- Green / brown: meconium — continuous CTG.
- Heavy blood: abruption concern.
- Foul smell: infection (chorioamnionitis).
Different scenarios
Scenario 1: Anomaly scan AFI 18 cm
Normal. Routine care continues.
Scenario 2: 38 weeks, AFI 4 cm
Oligohydramnios at term. Induction often recommended.
Scenario 3: 28 weeks GDM, AFI 28 cm
Polyhydramnios. Tight glucose control. Detailed scan to rule out fetal anomaly.
Scenario 4: Sudden gush at 36 weeks
Hospital. Confirm ROM. Induction within 24h usually.
Scenario 5: 32 weeks, severe oligohydramnios + IUGR
Steroids. Magnesium. Likely delivery within 24-48h.
Care guidance — amniotic fluid
- SDP preferred over AFI (fewer false-positive).
- Stay hydrated.
- Tell midwife of leaks / gushes.
- Monitor fetal movements.
- Polyhydramnios: GDM screen + anomaly scan.
- Oligohydramnios: growth + Doppler scans.
- Term oligohydramnios: induction often.
- Severe early oligohydramnios: specialist fetal medicine.
Sources
- NICE NG201. Antenatal care.
- SMFM. Amniotic fluid assessment.
- RCOG. Reduced fetal movements; PPROM Green-top.
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Frequently asked questions
What is amniotic fluid?
The fluid around baby in the womb. Produced mainly by baby's URINE (after 16 weeks) + lung secretions; resorbed by baby SWALLOWING + placenta. CONSTANTLY recycled — turns over every few hours. FUNCTIONS: cushions baby; allows movement (helps muscle/lung development); maintains temperature; protects umbilical cord from compression. VOLUME peaks ~34-36 weeks (~800 mL average) then gradually declines. MEASURED in pregnancy ultrasound to assess fetal well-being.
How is amniotic fluid measured?
TWO methods: (1) AMNIOTIC FLUID INDEX (AFI) — adds up DEEPEST POCKET in each of 4 quadrants of uterus. Normal 5-25 cm. (2) SINGLE DEEPEST POCKET (SDP / DVP) — largest vertical pocket without cord/limbs. Normal ≥2 cm. SDP NOW PREFERRED (NICE / SMFM) — AFI causes more false-positive interventions; SDP equivalent + simpler. SOMETIMES BOTH measured; usually one or the other. SUBJECTIVE assessment ('plenty', 'reduced') less accurate.
What's oligohydramnios?
LOW amniotic fluid. AFI ≤5 cm OR SDP <2 cm. CAUSES: (1) PLACENTAL insufficiency (commonest in late pregnancy) — placenta not delivering enough → baby makes less urine → less fluid; (2) ROM (waters broken / leaking); (3) FETAL renal problem (urinary tract obstruction, absent kidneys); (4) POST-DATES (placenta ageing); (5) MATERNAL DEHYDRATION; (6) MEDICATIONS (ACE inhibitors, NSAIDs). IMPLICATIONS: increased risk of cord compression, fetal distress, growth restriction, perinatal mortality. MANAGEMENT depends on cause + gestation.
What's polyhydramnios?
HIGH amniotic fluid. AFI ≥25 cm OR SDP >8 cm. CAUSES: (1) GESTATIONAL DIABETES (most common) — high maternal glucose → fetal polyuria; (2) FETAL anomalies — gut atresia/obstruction (can't swallow normally); neurological/skeletal issues impairing swallowing; (3) FETAL anaemia; (4) TWIN-TWIN transfusion syndrome; (5) INFECTION; (6) IDIOPATHIC (often). RISKS: preterm labour, malpresentation, placental abruption, cord prolapse, PPH. MANAGEMENT: glucose testing, anomaly scan, sometimes amnioreduction (large-volume amniocentesis).
What does low amniotic fluid mean for baby?
DEPENDS on cause + severity + gestation. MILD reduction at term: often managed expectantly; delivery if persistent. MODERATE/SEVERE: increased monitoring (CTG, BPP) ± delivery. AT TERM with oligohydramnios: induction usually recommended. PRETERM: balance risks; consider steroids; specialist surveillance. EARLY ONSET severe oligohydramnios (<24 wk): poor prognosis (lung development needs fluid — pulmonary hypoplasia). LATER ONSET (28+ wk): much better outcomes with monitoring + timely delivery.
What about high amniotic fluid for baby?
MILD: usually no issue; resolves often. MODERATE/SEVERE: investigate cause (GDM screening, detailed anomaly scan); risks of preterm labour, cord prolapse, malpresentation. AMNIOREDUCTION sometimes done if severe + symptomatic (uncomfortable, breathless). DELIVERY MANAGEMENT: avoid sudden ROM (cord prolapse risk); CONSIDER controlled rupture in theatre; INDUCTION timing balances risks. POSTNATAL: gut/anomaly checks if cause unclear.
Can I increase amniotic fluid by drinking more water?
MIXED EVIDENCE. MATERNAL DEHYDRATION can reduce fluid; correcting hydration may help mildly. STUDIES: 2L water/day for several days modestly improved AFI in some studies. NOT a substitute for medical evaluation. SAFE to drink more (don't overdo); always tell midwife if you're trying. AMNIOINFUSION (fluid added through cervix during labour) different procedure — sometimes used for variable decelerations from cord compression.
When is amniotic fluid measured?
ROUTINELY at: 20-WEEK ANOMALY SCAN (subjective assessment); GROWTH SCANS (28, 32, 36 wk for high-risk); BIOPHYSICAL PROFILE; INDUCED labour assessment; CONCERNED about reduced movements / decreasing fundal height. ADDITIONAL: post-dates (≥41 wk); GDM monitoring; PE/IUGR surveillance; suspected ROM. NOT routine in low-risk uncomplicated pregnancies.
What's PROM / PPROM?
PROM = Premature Rupture of Membranes — waters break BEFORE labour starts at term. PPROM = Preterm PROM — waters break BEFORE 37 weeks. SIGNS: sudden gush of fluid; ongoing leak; wet pad/underwear. IF SUSPECTED: hospital assessment same day. CONFIRMATION: sterile speculum exam ± nitrazine test, amniotic fluid ferning, AmniSure. PROM at term: induce within 12-24h usually. PPROM 24-34 wk: antibiotics + steroids + watchful waiting; deliver if infection, distress, growth issues; aim 36-37 weeks unless complications.
What if my fluid is just slightly low?
BORDERLINE oligohydramnios (AFI 5-8 cm OR SDP 2-3 cm): increased monitoring; hydration; growth scan + Doppler; sometimes delivery at term. TERM (37+): induction often recommended. PRETERM borderline: depends on cause + gestation + baby's well-being. NOT all 'low' fluid means concerning — needs CONTEXT (baby growing well? Movements good? Doppler normal?).
Does amniotic fluid colour matter?
AT BIRTH / ROM: CLEAR / straw / pale pink = normal. GREEN / brown = MECONIUM (baby's first stool passed in utero) — sign of fetal stress in some cases; can also be normal at term; CONTINUOUS CTG advised. BLOODY pink streaks normal. HEAVY blood = abruption or other concern. FOUL SMELL = infection (chorioamnionitis) — needs antibiotics + delivery. BROWN = old meconium or stale fluid.
What's amnioinfusion?
PROCEDURE during labour where saline is infused into uterus through a catheter through the cervix. INDICATIONS: persistent VARIABLE DECELERATIONS from cord compression (especially with oligohydramnios); some units use for thick meconium (controversial). RELIEVES cord pressure; can improve CTG. NOT widely used in UK NHS; more US practice. RISKS: infection, uterine overdistension (rare). SHOULDN'T DELAY delivery if otherwise indicated.
Can I test waters break at home?
NOT RELIABLY. SOME home test strips claim to detect amniotic fluid (pH-based) — UNRELIABLE; false positives from blood, semen, infections. GO TO MATERNITY UNIT if suspecting ROM — sterile speculum + tests confirm. SIGNS at home: SUDDEN gush; ONGOING wetness regardless of position; SOAKED PADS in <1 hour; CLEAR / pale liquid. URINE different — controllable, smell. WHEN UNSURE — call for advice.
What's anhydramnios?
NO measurable amniotic fluid. RARE; severe. CAUSES: fetal kidney problems (Potter sequence — bilateral renal agenesis); severe placental insufficiency; PPROM with complete drainage. IMPLICATIONS: pulmonary hypoplasia (lungs don't develop without fluid); skeletal contractures (limbs not moving); often FATAL especially if early-onset. AMNIOINFUSION TRIAL sometimes attempted (RAFT trial 2023) — adds fluid; preliminary evidence for some cases of renal agenesis. SPECIALIST fetal medicine team essential.
How does this relate to other calculators on BumpBites?
Companion: /calculators/biophysical-profile (BPP includes fluid); /calculators/fetal-weight; /calculators/preeclampsia-diagnosis (placental insufficiency); /calculators/gdm-ogtt (polyhydramnios risk); /calculators/kick-counter; /calculators/contraction-timer; /calculators/membrane-sweep.