Either obstruction of the fetal urinary tract or renal agenesis will lead to oligohydramnios not polyhydramnios. Common associations of polyhydramnios: CNS malformations: Anencephaly Spina bifida GIT malformations: Esophageal atresia Tracheoesophageal fistula Duodenal atresia Pyloric stenosis Nonimmune hydrops Less common associations: Fetal pseudohypoaldosteronism Fetal Bater or hyperprostaglandin E syndrome Fetal nephrogenic diabetes insipidus Placental chorioangioma Fetal sacrococcygeal teratoma Maternal substance abuse Idiopathic hydramnios is excess amniotic fluid not associated with congenital anomalies, malformations, maternal diabetes, isoimmunization, infection, tumors or multifetal gestation. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 21. Disorders of Amnionic Fluid Volume. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.