[Estrogens. Contraceptive therapy]

To evaluate the competitive molecular phenomenon of Ethynyl-Estradiol (EE-2) from contraceptive formulations against the endogenous Estradiol (E-2) at the intra and the extracellular compartments, plasma and endometrial samples were simultaneously obtained on different days of the pseudomenstrual cycle from oral contraceptive users under EE-2+ Norgestrel (30 micrograms/+ 500 micrograms) and EE-2+ Norethindrone (50 micrograms + 1.0 mg) in order to quantify EE-2 & E-2. When measuring both molecules it was shown that the chronic administration of steroids regardless of the pharmacological action of the progestin component the lower content of EE-2 (30 micrograms) does not compete substantially at the circulating level permitting the cyclic fashion of the natural estradiol while at the endometrial compartment such phenomenon is not seen thus, a local infertility effect should be reconsidered to anticipate a different approach in the future of contraception.