In a newborn infant with esophageal stenosis and atresia extending 9 cm, the amniotic fluid volume was found to be normal, yet this infant could not possibly have swallowed. In a case of anencephaly in which the infant had partial esophageal atresia and stenosis extending 1 cm, the infant swallowed sodium diatrizoate (Hypaque Sodium) injected into the amnion, yet the mother had polyhydramnios. Two more cases of anencephaly are presented in which the infants swallowed radiopaque media, yet both mothers had polyhydramnios.
These findings emphasize that polyhydramnios is a complex phenomenon and indicate that "failure to swallow" cannot explain all such cases associated with esophageal atresia and anencephaly.
John Nichols, Rosemary Schrepfer. Polyhydramnios in Anencephaly. JAMA, Aug 15, 1966, Vol 197, No 7
It is apparent that a delicate homeostatic mechanism exists which balances amniotic fluid formation and résorption to maintain a normal volume of approximately 1,000 ml at term. Polyhydramnios results when the rate of formation at all sites exceeds the rate of résorption.