DEVELOPMENT OF AF AND THE AF CIRCULATION
A fluid-filled extracelomic cavity which will eventually become the
amnionic space is identified near the time of implantation, even
before the embryo is recognizable. During embryogenesis, AF
volume increases faster than embryonic size. The water in AF
originally comes from maternal plasma and passes through the
fetal membranes based on hydrostatic and osmotic forces. As the
placenta and fetal vessels develop, water and solute from maternal
plasma pass across the placenta to the fetus and then to the AF. In
the early fetal period, AF volume and fetal size are related in a
linear fashion. AF volume increases from about 25 ml at 10 weeks
to about 400 ml at 20 weeks. During this period, AF composition is
similar to fetal plasma. There is rapid bi-directional diffusion
between the fetus and the AF across the not-yet-keratinized fetal