Embryology · Fetal period, placenta and twins
Amniotic fluid: polyhydramnios and oligohydramnios !
- Topic
The fetus lives in amniotic fluid. From mid-pregnancy that fluid is mostly fetal urine, which the fetus swallows again.
Updated 2026-11-01
See it in 3D: Kidneys · Esophagus
What it is
Amniotic fluid protects the fetus, lets it move and is essential for lung growth. From the second trimester its main source is fetal urine, and it is removed mostly by fetal swallowing. So too much fluid (polyhydramnios) points to problems with swallowing or absorbing, and too little (oligohydramnios) to problems with the kidneys, membranes or placenta.
Key facts
- Roles: cushions blows, prevents amniotic adhesions, keeps temperature steady and allows movement and fetal breathing.
- Fetal breathing movements draw fluid into the lungs and stimulate their growth.
- The fluid is slightly alkaline: it turns nitrazine paper and dries in a «fern» pattern, signs used to confirm ruptured membranes.
- Amniocentesis samples the fluid to study karyotype, infection or lung maturity.
Polyhydramnios and oligohydramnios
| Feature | Polyhydramnios | Oligohydramnios |
|---|---|---|
| Mechanism | Fetus cannot swallow or absorb | Fetus does not pass urine, or fluid leaks |
| Fetal causes | Esophageal or duodenal atresia, anencephaly | Renal agenesis, urinary obstruction |
| Maternal or placental causes | Diabetes, recipient twin | Ruptured membranes, placental insufficiency, ACE inhibitors |
| Consequences | Preterm birth, cord prolapse | Pulmonary hypoplasia, deformities, cord compression |
Clinical notes
- Polyhydramnios on ultrasound with a small or absent fetal stomach: suspect esophageal atresia.
- «Double bubble» with polyhydramnios: suspect duodenal atresia.
- Oligohydramnios with an empty fetal bladder and no visible kidneys: suspect bilateral renal agenesis.
- Rupture of membranes before viability can cause pulmonary hypoplasia.
Test yourself
Think of the answer, then tap the question to see it.
What is the main source of amniotic fluid in the second half of pregnancy?
Fetal urine; the fluid is removed mainly when the fetus swallows it and absorbs it in the gut.
Which gut malformation is suspected with polyhydramnios and no visible fetal stomach?
Esophageal atresia: the fetus cannot get swallowed fluid to the stomach and intestine.
Which sequence results from severe oligohydramnios due to bilateral renal agenesis?
The Potter sequence: flattened face, limb deformities and pulmonary hypoplasia.
Why does prolonged oligohydramnios harm the fetal lung?
Without fluid the chest is compressed and the lung loses the fluid and distension it needs to branch and grow.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Fetal membranes: amnion, chorion, yolk sac and allantois
- Esophageal atresia and tracheoesophageal fistula
- Duodenal atresia and stenosis
- Renal agenesis and the Potter sequence
- Pulmonary hypoplasia
- Twins: dizygotic and monozygotic, chorionicity, twin–twin transfusion
- Neural tube defects: anencephaly, spina bifida and encephalocele
- ACE inhibitors and ARBs: fetal kidney damage
Related conditions
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Amniotic fluid: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2024); Polyhydramnios: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2025); Potter syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2025); Langman's Medical Embryology, 13th ed., ch. 8: Third Month to Birth: The Fetus and Placenta (Wolters Kluwer, 2015).
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Representative model, for educational purposes only.