Obstetric Topic 33. Amniotic fluid and its disorders. Polyhydramnios and oligohydramnios

I. Amniotic Fluid

Definition and Dynamics

• Early pregnancy: maternal plasma dialysate diffuses across amnion/chorion and fetal skin.
• Second trimester onward: fetal urine + fetal lung secretion are main sources.
• Elimination: mainly fetal swallowing; also intramembranous transfer.
• Volume: peaks around 28-32/32-36 weeks at about 1000 mL; term volume about 500-800 mL.

Functions

• Cushions fetus against external trauma.
• Prevents cord compression.
• Supports fetal breathing movements and lung development.
• Allows fetal movement → musculoskeletal development.
• Supports fetal swallowing → gastrointestinal development.
• Antibacterial/anti-inflammatory properties → infection protection.

Ultrasound Assessment

• Single deepest pocket / maximum vertical pocket (SDP/MVP): deepest vertical pocket without cord or fetal parts.
• Normal SDP: 2-8 cm.
• Amniotic fluid index (AFI): sum of deepest vertical pockets in 4 uterine quadrants.
• Normal AFI: about 6-24 cm.

II. Polyhydramnios

Definition and Causes

• Polyhydramnios / hydramnios: excessive amniotic fluid for gestational age.
• Diagnostic thresholds: volume >2000 mL, AFI >24 cm or SDP >8 cm.
• Maternal diabetes: fetal hyperglycemia → osmotic diuresis/polyuria.
• Fetal swallowing impairment: anencephaly, neural tube/CNS defects, esophageal or GI atresia.
• Fetal hydrops/anemia.
• Multiple pregnancy / TTTS: recipient twin develops polyhydramnios.
• Idiopathic: common, especially mild cases.

Clinical Features and Evaluation

• Large-for-dates uterus, rapid abdominal enlargement, fetus difficult to palpate/hear.
• Severe cases: maternal dyspnea, abdominal discomfort, uterine irritability or preterm contractions.
• Work-up:
  1. Ultrasound: increased AFI/SDP, fetal anomalies, hydrops, growth and presentation.
  2. Screen for diabetes.
  3. Consider infection/genetic/anemia work-up if severe, anomalous or unexplained.

Complications and Management

• Complications: preterm labor, PROM/PPROM, placental abruption, malpresentation, cord prolapse and postpartum hemorrhage from uterine atony.
• Treatment: manage underlying cause, especially diabetes, fetal anemia, infection or TTTS.
• Mild idiopathic polyhydramnios: observation.
• Severe symptomatic polyhydramnios: decompression amniocentesis / amnioreduction.
• Delivery: according to cause, severity, fetal presentation and obstetric indications.

III. Oligohydramnios

Definition and Causes

• Oligohydramnios: decreased amniotic fluid for gestational age.
• Diagnostic thresholds: volume <500 mL, AFI <5-6 cm or SDP <2 cm.
• PROM/PPROM: acute fluid loss; must always be excluded.
• Fetal renal/urinary causes: renal agenesis, urinary tract obstruction, severe renal disease.
• Uteroplacental insufficiency: preeclampsia, hypertension, placental disease.
• FGR/IUGR and postterm pregnancy.
• TTTS: donor twin has oligohydramnios.
• Drugs: ACE inhibitors/ARBs and NSAIDs/prostaglandin synthesis inhibitors.

Clinical Features and Evaluation

• Small-for-dates uterus; fetal parts easily palpable.
• Work-up:
  1. Rule out PROM with history and sterile speculum testing if needed.
  2. Ultrasound: decreased AFI/SDP, fetal kidneys/bladder/urinary tract, fetal anomalies and IUGR.
  3. Assess growth, placenta and Doppler if uteroplacental insufficiency is suspected.
  4. Umbilical artery Doppler if FGR/placental insufficiency suspected.
  5. Consider fetal genetic testing if anomalies or aneuploidy signs.
  6. Review maternal disease and drugs.

Complications and Management

• Complications: dystocia, FGR/IUGR, cord compression, fetal distress in labor and fetal death.
• Early severe oligohydramnios: pulmonary hypoplasia and fetal compression/deformations.
• Treatment: correct cause if possible, stop offending drugs, treat maternal disease and manage PROM/FGR according to separate protocols.
• Maternal hydration: may transiently increase fluid; useful mainly as supportive therapy.
• Surveillance: fetal testing, fluid and growth follow-up if not delivering.
• Intrapartum amnioinfusion: isotonic fluid during labor for recurrent variable decelerations from cord compression.