Ethynodiol diacetate is a synthetic progestin primarily used in hormonal contraceptives. It functions by preventing ovulation, thickening cervical mucus to reduce sperm penetration into the uterus, and altering the uterine lining to make it less receptive to implantation of a fertilized egg. It is commonly combined with an estrogen in oral birth control pills but can also be used alone in progestin-only formulations. Its chemical stability and good oral bioavailability support daily dosing in contraceptive regimens. Additionally, it is employed in hormone replacement therapies to regulate menstrual cycles and alleviate symptoms of hormonal imbalances.