Figure 5.

Saturation/escape model of transfer of glucocorticoid from mother to fetus through the placenta to support normal fetal lung development. The black vertical arrows represent the amount of corticosterone transferred in the morning (thin arrow) or the evening (thick arrow). Placental 11-β hydroxysteroid dehydrogenase limits the amount of active corticosterone that is transferred to the fetus. High levels of corticosterone are proposed to saturate the enzyme, to escape inactivation, and to be transferred to the fetus. Fetal corticosterone is derived both from that transferred from the mother and that secreted from the fetal adrenal and supports the development of fetal organs, including lung.