Twenty-four non-pregnant women 18-35 years of age, who had not been on hormonal contraception for at least 2 months prior to the study or who were least 3 months postpartum and non-lactating, were enrolled in this study between October 2003 and February 2005. The study protocol was approved by the University of Vermont Institutional Review Board, and all participants gave written informed consent. Exclusion criteria for participation were personal or family history of venous thromboembolism or coagulation disorders, pregnancy, uncontrolled hypertension, cardiovascular disease, complicated migraine headaches, breast cancer, diabetes, abnormal uterine bleeding, liver disease, or desire for pregnancy in less than 6 months.