skin and the surfaces of the amnion, placenta, and umbilical
cord, each being freely permeable to water and solutes. During
this phase of pregnancy, the AF serves both as a physiologic buffer
and an extension of the fetal extracellular compartment. By 8
weeks of gestation, the urethra is patent and the fetal kidneys
make urine. Shortly thereafter fetal swallowing begins; however,
neither fetal urination nor swallowing contributes significantly
to the content or volume of AF until the second half of pregnancy.
Keratinization of fetal skin begins at 19 to 20 weeks of gestation
and is usually complete at 25 weeks after conception. When
keratinization is complete, the relationship between fetal size
and AF volume is no longer linear. By 28 weeks of gestation,
AF volume reaches a volume of B800 ml where it plateaus
near term gestation and thereafter declines to B400 ml at 42
weeks.1