It is intriguing to ask what the evolutionary function of
the development of this insensitivity may be. The most
plausible explanation of the strong correlation between
maternal and fetal cortisol levels [8,9] is direct transfer
across the placenta. Cortisol crosses membranes easily,
and as the maternal fetal ratio is over 10/1 this is compatible
with considerable metababolism of cortisol as it crosses the placenta. If cortisol from the mother can affect
fetal brain development [8], and possibly time of parturition,
it may be important to buffer the fetus from these effects,
especially in later pregnancy