Polyhydramnios & Oligohydramnios
Obstetrics & Gynaecology · High-risk Pregnancy · lean revision notes
Polyhydramnios & Oligohydramnios
Amniotic fluid disorders are a cornerstone of high-risk pregnancy. Too much fluid (polyhydramnios) points towards fetal swallowing defects or maternal diabetes; too little (oligohydramnios) points towards fetal urination defects, rupture of membranes, or placental insufficiency. The USG cut-offs (AFI, SDP) and the classic anomaly associations are repeatedly tested.
Amniotic fluid: physiology in brief
Amniotic fluid volume (AFV) reflects a dynamic balance between production and removal:
- Production (inflow): fetal urine (the dominant source after ~20 weeks) and lung fluid secretion.
- Removal (outflow): fetal swallowing (major route) and intramembranous absorption across the fetal surface of the placenta.
Volume peaks at roughly 800–1000 mL around 34–36 weeks, then declines towards term. After 28 weeks the fluid is essentially fetal urine, which is why renal and lower urinary tract anomalies cause oligohydramnios, while swallowing/GI obstruction causes polyhydramnios.
High-yield: After 20 weeks, fetal urine is the main source of amniotic fluid and fetal swallowing is the main route of removal. This single physiological fact explains almost every anomaly association you will be asked about.
How amniotic fluid is measured
Two semi-quantitative ultrasound techniques are used. Know both definitions and their thresholds cold.
| Method | Technique | Normal range | Definition matters for |
|---|---|---|---|
| Amniotic Fluid Index (AFI) | Uterus divided into 4 quadrants; deepest vertical pocket (free of cord/fetal parts) in each is summed | 5–24 cm (some use 5–25) | Singleton pregnancy assessment |
| Single Deepest Pocket (SDP) / Maximum Vertical Pocket (MVP) | Single deepest cord- and limb-free vertical pocket | 2–8 cm | Twins; preferred per recent evidence |
Cut-offs to memorise
| Condition | AFI | SDP / MVP |
|---|---|---|
| Oligohydramnios | < 5 cm | < 2 cm |
| Normal | 5–24 cm | 2–8 cm |
| Polyhydramnios | > 24 cm (≥25) | > 8 cm |
| Borderline/low normal | 5–8 cm | — |
High-yield: AFI < 5 cm OR SDP < 2 cm = oligohydramnios. AFI > 24 cm OR SDP > 8 cm = polyhydramnios. These four numbers are the single most repeated recall point on this topic.
High-yield: In twin/multiple gestation, the SDP (MVP) is preferred over AFI because dividing a uterus with two sacs into four quadrants is unreliable. SDP is also favoured generally as it reduces over-diagnosis of oligohydramnios and unnecessary inductions.
POLYHYDRAMNIOS
Definition & grading
Polyhydramnios = AFV > ~2000 mL, or AFI > 24 cm, or SDP > 8 cm. Severity grading by AFI:
| Grade | AFI | SDP |
|---|---|---|
| Mild | 25–29.9 cm | 8–11 cm |
| Moderate | 30–34.9 cm | 12–15 cm |
| Severe | ≥ 35 cm | ≥ 16 cm |
Mild polyhydramnios (~80% of cases) is usually idiopathic. The more severe the polyhydramnios, the higher the chance of an identifiable fetal anomaly.
Etiology & pathophysiology
The mechanism is increased fluid production OR decreased fetal swallowing/absorption.
1. Idiopathic (~60–70%) — diagnosis of exclusion.
2. Maternal causes
- Diabetes mellitus (gestational & pregestational): the commonest identifiable cause. Fetal hyperglycaemia → osmotic polyuria → polyhydramnios. Always check maternal glucose.
- Multiple gestation, especially the recipient twin in twin-to-twin transfusion syndrome (TTTS).
3. Fetal causes — "cannot swallow / GI obstruction"
- Oesophageal atresia (with or without TEF) — classic. Fetus cannot swallow fluid. USG: absent or small stomach bubble + polyhydramnios.
- Duodenal atresia — "double bubble" sign (dilated stomach + proximal duodenum); strong association with Down syndrome (trisomy 21).
- Anencephaly / neural tube defects → impaired swallowing + transudation.
- Diaphragmatic hernia, facial clefts, micrognathia.
- Polyuric states: Bartter syndrome, fetal anaemia (high-output), high-output cardiac failure.
4. Fetal hydrops / immune & non-immune causes (anaemia, infection e.g. parvovirus B19, CMV, syphilis; chromosomal anomalies).
High-yield mnemonic for polyhydramnios causes — "DIABETIC GUT": Diabetes, Idiopathic, Anencephaly, Blockage of gut (oesophageal/duodenal atresia), Edwards/Down (aneuploidy), TTTS (recipient), Infection, Cardiac high-output; GUT = swallowing/GI defects.
Clinical features
- Rapid uterine enlargement; fundal height > period of amenorrhoea (size > dates).
- Difficulty palpating fetal parts, distant/muffled fetal heart sounds, fluid thrill.
- Maternal dyspnoea, abdominal discomfort, oedema of legs/vulva (IVC compression), tense shiny abdomen in acute cases.
- Acute polyhydramnios (rare, mid-trimester, often monochorionic twins/TTTS) presents with sudden painful distension.
Diagnosis
Clinical suspicion (size > dates) → USG (AFI/SDP confirmation + grading) → search for cause.
Workup: detailed anomaly scan (stomach bubble, double bubble, NTD, cardiac), maternal OGTT/glucose, blood group & antibody screen (Rh), TORCH/parvovirus serology, fetal karyotype/cfDNA if anomaly or aneuploidy suspected, MCA Doppler for fetal anaemia.
Complications
- Maternal: preterm labour, malpresentation, cord prolapse, placental abruption (esp. after sudden decompression), uterine atony → postpartum haemorrhage, respiratory compromise.
- Fetal: prematurity, related to underlying anomaly, increased perinatal mortality.
- At ROM: sudden gush risks cord prolapse and abruptio placentae (due to rapid uterine decompression).
Management
- Treat the cause: glycaemic control in diabetes; laser photocoagulation for TTTS.
- Mild/moderate, asymptomatic: expectant, serial USG and antenatal surveillance.
- Symptomatic / severe:
- Amnioreduction (therapeutic amniocentesis): remove fluid slowly to relieve maternal distress; risk of preterm labour/abruption if too rapid.
- Indomethacin: reduces fetal urine output. Avoid after 32 weeks (risk of premature closure of ductus arteriosus and oligohydramnios). Contraindicated in oligohydramnios states.
- Steroids for lung maturity if preterm delivery anticipated; deliver in a centre equipped for neonatal surgery if GI anomaly.
- Active management of third stage and readiness for PPH (oxytocics).
High-yield: Absent stomach bubble + polyhydramnios = oesophageal atresia. Double bubble + polyhydramnios = duodenal atresia (think trisomy 21).
OLIGOHYDRAMNIOS
Definition
Oligohydramnios = AFV reduced, AFI < 5 cm or SDP < 2 cm. Anhydramnios = virtually no fluid. Borderline/low AFI = 5–8 cm.
Etiology & pathophysiology
Mechanism = decreased production (urine) OR increased loss (ROM). Best remembered by trimester.
Fetal causes — "cannot make/pass urine":
- Bilateral renal agenesis (Potter syndrome) — no kidneys, no urine → severe early oligohydramnios/anhydramnios; lethal.
- Posterior urethral valves (PUV) — bladder outlet obstruction in male fetus → distended bladder + "keyhole sign", hydronephrosis.
- Bilateral multicystic dysplastic/polycystic kidneys (e.g. ARPKD — large echogenic kidneys).
- Other obstructive uropathies (ureteropelvic junction obstruction).
Placental/maternal causes:
- Uteroplacental insufficiency / IUGR / pre-eclampsia — redistribution of fetal blood away from kidneys → reduced renal perfusion → reduced urine.
- Post-term pregnancy (>42 wk) — declining placental function.
- Maternal dehydration, ACE inhibitors/ARBs and NSAIDs/indomethacin (drug-induced).
Membrane cause:
- PPROM (preterm premature rupture of membranes) — commonest cause of acute oligohydramnios; confirm leakage.
High-yield mnemonic — "DRIPPC": Drugs (ACEi/ARB, NSAID), Renal agenesis/PUV, IUGR/placental insufficiency, PROM, Post-term, Chromosomal. (Causes of oligohydramnios.)
Potter sequence
Severe early/prolonged oligohydramnios (classically bilateral renal agenesis) → mechanical fetal compression:
No urine → no fluid → fetal compression → the Potter facies + pulmonary hypoplasia + limb deformities.
- Potter facies: low-set ears, flattened nose, recessed chin, infraorbital creases, hypertelorism.
- Pulmonary hypoplasia — the actual cause of neonatal death (fluid needed for lung development).
- Limb contractures/positional deformities (e.g. clubfoot), amnion nodosum.
High-yield: In Potter sequence the cause of death is pulmonary hypoplasia, not the renal lesion itself. Amniotic fluid is essential for lung canalicular development in the 2nd trimester.
Clinical features
- Size < dates; uterus feels small and compact, fetal parts easily palpable, fetus feels "moulded."
- Reduced fetal movements; on USG fetus appears crowded with scanty fluid.
Diagnosis & investigation
Confirm small for dates → USG (AFI < 5 / SDP < 2) → exclude ROM (sterile speculum, pooling, nitrazine, AmniSure/PAMG-1) → fetal anatomy (kidneys, bladder, keyhole sign) → Doppler (umbilical artery for IUGR/insufficiency).
- Visualise fetal bladder and kidneys — absent bladder filling over time suggests renal cause.
- Umbilical artery Doppler (absent/reversed end-diastolic flow → severe placental insufficiency).
- Check maternal drug history (ACEi/ARB, NSAID), BP and proteinuria (pre-eclampsia).
Complications
- Pulmonary hypoplasia (early, prolonged).
- Limb contractures, positional deformities from compression.
- Cord compression → variable decelerations, fetal distress, meconium aspiration.
- IUGR, oligohydramnios-related stillbirth.
Management
- Identify & treat cause: stop offending drugs (ACEi/ARB, NSAID); maternal hydration can transiently improve AFI; manage pre-eclampsia/IUGR.
- PPROM: depends on gestation — steroids, latency antibiotics (e.g. erythromycin/ampicillin), and timed delivery.
- Amnioinfusion: intrapartum, for recurrent variable decelerations from cord compression; also diagnostic to visualise anatomy.
- Surveillance & delivery: isolated oligohydramnios at/near term → induction/delivery, as AFI < 5 is associated with adverse perinatal outcome. Lethal anomalies (bilateral renal agenesis) → counselling regarding poor prognosis.
High-yield: Amnioinfusion is the intrapartum remedy for variable decelerations caused by cord compression in oligohydramnios.
Swallowing vs Urination — the master comparison
This single table is the most examined concept; commit it to memory.
| Defect type | Effect on fluid | Classic conditions | Buzz signs |
|---|---|---|---|
| Cannot swallow / GI obstruction | Polyhydramnios | Oesophageal atresia, duodenal atresia, anencephaly | Absent stomach bubble; double bubble (trisomy 21) |
| Cannot urinate / renal defect | Oligohydramnios | Bilateral renal agenesis (Potter), PUV, ARPKD | Keyhole sign (PUV); empty bladder; Potter facies |
| Increased urine output | Polyhydramnios | Maternal diabetes, Bartter, fetal anaemia | Macrosomia (diabetes) |
| Membrane rupture | Oligohydramnios | PPROM | Pooling, +nitrazine, +PAMG-1 |
High-yield: Swallowing defect → polyhydramnios; urination defect → oligohydramnios. Decide which "tap" (inflow/outflow) is broken and the answer follows.
Key differentials & traps
- Size > dates: polyhydramnios, multiple gestation, macrosomia (diabetes), wrong dates, molar/fibroid.
- Size < dates: oligohydramnios, IUGR, wrong dates, transverse lie, oligo + IUGR coexist in placental insufficiency.
- Double bubble = duodenal atresia (Down syndrome); absent bubble = oesophageal atresia. Don't swap these.
- Keyhole sign in a male fetus with oligohydramnios = posterior urethral valves.
- Indomethacin: treats polyhydramnios (↓ urine) but causes oligohydramnios and ductal closure after 32 weeks — a classic two-edged fact.
- ACE inhibitors/ARBs in pregnancy → fetal renal failure, oligohydramnios, skull hypoplasia (contraindicated).
Recently asked / exam angle
- Recall of AFI/SDP cut-offs (oligo < 5 / < 2; poly > 24 / > 8) — single most frequent stem.
- Vignette: polyhydramnios + absent stomach bubble → answer oesophageal atresia; + double bubble → duodenal atresia / trisomy 21.
- Vignette: bilateral renal agenesis → Potter sequence, and "cause of neonatal death = pulmonary hypoplasia."
- Oligohydramnios + male fetus + dilated bladder/keyhole → posterior urethral valves.
- Preferred index in twins = SDP/MVP (not AFI).
- Maternal diabetes as the commonest identifiable maternal cause of polyhydramnios.
- Drug associations: indomethacin → poly relief but ductal closure/oligo after 32 wk; ACEi/ARB → oligohydramnios.
- Amnioinfusion for cord-compression variable decelerations; amnioreduction/indomethacin for symptomatic polyhydramnios.
- Complications at ROM in polyhydramnios: cord prolapse, abruption, PPH (atony).
Rapid revision
- After 20 weeks, fetal urine makes the fluid; fetal swallowing removes it.
- Oligohydramnios: AFI < 5 cm or SDP < 2 cm. Polyhydramnios: AFI > 24 cm or SDP > 8 cm.
- Normal AFI 5–24 cm; normal SDP 2–8 cm; use SDP/MVP in twins.
- Polyhydramnios = can't swallow / overproduces urine → diabetes, oesophageal/duodenal atresia, anencephaly.
- Oligohydramnios = can't urinate / loses fluid → renal agenesis, PUV, IUGR, PPROM, post-term.
- Absent stomach bubble = oesophageal atresia; double bubble = duodenal atresia (Down syndrome).
- Keyhole sign (male fetus) = posterior urethral valves.
- Potter sequence: renal agenesis → oligo → pulmonary hypoplasia (cause of death) + Potter facies + limb deformities.
- Indomethacin reduces fetal urine (treats poly) but avoid > 32 wk — ductal closure & oligohydramnios.
- ACE inhibitors/ARBs in pregnancy cause oligohydramnios and fetal renal injury.
- Amnioreduction for symptomatic polyhydramnios; amnioinfusion for cord-compression decelerations.
- Polyhydramnios at delivery → watch for cord prolapse, abruption, and PPH from uterine atony.