Etiology

Increased production (most commonly maternal diabetes) or decreased clearance (obstruction or poor swallowing). Most common causes are: (i) maternal diabetes (20-30%), (ii) fetal malformations (10-15%), (iii) multiple gestations (5%), Rh or other isoimmunization, “Mirror syndrome,” others; unknown cause (about 50%, especially for mild polyhydramnios). Severe polyhydramnios is usually pathologic, not idiopathic.