Ethinyl estradiol is the active estrogen component in all presently used birth control pills.
Credits: C. Matthew Peterson M.D.
The ethinyl group at position 17 of estradiol allows estrogen to be orally absorbed. Mestranol, also shown, adds the methylester group at position 3. Because this methylester group is cleaved in the metabolism of mestranol, ethinyl estradiol is essentially the estrogen present in all birth control pills. Over the last 40 years, there have been four generations of products each of which improved the safety of this product!