Steroids in human intrauterine fluids of early pregnancy

Coelomic fluid contains relatively high concentrations of progesterone, oestradiol-17 beta and 17 alpha-hydroxyprogesterone which may be synthesized locally. Amniotic fluid contains lower concentrations of steroids (other than progesterone) than are found in coelomic fluid or maternal serum. Free diffusion of steroids across the amnion appears limited. This may constitute a mechanism to protect the embryo from unwanted exposure to biologically active steroids.