Uterine perfusion also impacts AF volume. Maternal dehydration
leads to increased fetal plasma osmolality and increased fetal
production of arginine vasopressin. This causes an increase in the
osmolality of both fetal urine and to a lesser extent AF. When
arginine vasopressin is injected directly into ovine AF, fetal urine
and AF osmolality increase and fetal urine output decreases
significantly, and yet AF volume does not change suggesting reverse
intramembranous flow from the isotonic fetal circulation to the
hypertonic AF.10
The composition of AF changes with gestational age. In the
second half of pregnancy, there is a decrease in sodium and
chloride concentrations, an increase in urea and creatinine
concentrations, and an overall decrease in AF osmolality. Many
studies suggest that AF composition is more highly regulated than
AF volume. Wintour and Shandley4 has an excellent summary of
these studies.