What causes hydramnios?

Causes of Hydramnios

Hydramnios (polyhydramnios) results primarily from maternal diabetes mellitus and fetal anomalies that disrupt normal amniotic fluid regulation, though approximately 50-70% of mild cases remain idiopathic. 1

Major Etiological Categories

Maternal Causes

• Gestational diabetes mellitus is one of the two most common pathologic causes of polyhydramnios, accounting for approximately 25-33% of cases when systematically screened 1, 2, 3
• Pre-existing diabetes requiring insulin can present with polyhydramnios as the primary indicator of metabolic disturbance, occurring in approximately 8% of acute hydramnios cases 2

Fetal Structural Anomalies

Fetal anomalies represent the other most common pathologic cause and typically involve disruption of normal swallowing mechanisms 4, 1:

• Gastrointestinal obstructions including esophageal atresia, duodenal atresia, jejunal atresia, midgut volvulus, and malrotation prevent normal amniotic fluid swallowing 5
• Central nervous system anomalies that impair swallowing reflexes 1
• Thoracic abnormalities including congenital pulmonary airway malformations (CPAM), which cause polyhydramnios in approximately 5% of affected fetuses through esophageal compression from mediastinal shift 5
• Diaphragmatic hernia can obstruct venous return and compress the esophagus 5

Fetal Cardiac Abnormalities

• Cardiac arrhythmias, particularly tachyarrhythmias, cause polyhydramnios in approximately 8% of acute hydramnios cases through high-output cardiac failure 2
• Structural cardiac defects leading to congestive heart failure 5
• Cardiac rhabdomyomas associated with tuberous sclerosis can cause obstruction and cardiac failure 5

Multiple Gestation Complications

• Twin-twin transfusion syndrome (TTTS) affects 8-12% of monochorionic-diamniotic twin pregnancies, with the recipient twin developing polyhydramnios due to hypervolemia and polyuria 5
• The diagnostic criterion requires a maximal vertical pocket >8 cm in the recipient sac combined with <2 cm in the donor sac 5
• Twin pregnancies with acute hydramnios typically present earlier (average 27 weeks) compared to singletons (average 30 weeks) 2

Fetal Infections

• Parvovirus B19 infection causes transient fetal anemia and high-output cardiac failure, leading to non-immune hydrops fetalis with associated polyhydramnios 5
• Other congenital infections can disrupt normal fluid regulation 4, 1

Chromosomal and Genetic Abnormalities

• Chromosomal anomalies are found in approximately 13% of pregnancies with acute hydramnios 2
• Bartter syndrome (particularly types 1 and 2) causes severe fetal polyuria leading to maternal polyhydramnios, often requiring serial amniocenteses 5

Non-Immune Hydrops Fetalis (NIHF)

NIHF with polyserositis occurs in approximately 8% of acute hydramnios cases and has multiple causes 2:

• Chylothorax from lymphatic obstruction is the most common cause of isolated pleural effusion leading to NIHF 5
• Fetal tumors including sacrococcygeal teratomas, hemangiomas, lymphangiomas, and neuroblastomas cause high-output cardiac failure 5
• Hepatic disorders such as cirrhosis, hepatic necrosis, and cholestasis lead to hypoproteinemia 5
• Congenital nephrotic syndrome causes hypoproteinemia and NIHF 5

Placental and Umbilical Cord Abnormalities

• Chorioangiomas, umbilical cord angiomyxomas, umbilical artery aneurysms, cord vein thrombosis, umbilical vein torsion, true knots, and amniotic bands 5

Alloimmunization

• Red blood cell alloimmunization (Rh incompatibility) causes immune hydrops fetalis with polyserositis in approximately 3% of cases 2

Clinical Pitfalls

A critical caveat: When polyhydramnios is detected before 24 weeks gestation, nearly 50% of cases have chromosomal abnormalities with very poor survival rates 6. This contrasts sharply with later presentations where idiopathic causes predominate.

Important distinction: Approximately 50-70% of mild polyhydramnios cases remain idiopathic after thorough evaluation, particularly when diagnosed in the third trimester 1, 7. However, systematic screening for diabetes (including gestational diabetes testing) and detailed fetal anatomic survey are mandatory before assigning an idiopathic diagnosis 1, 2.