- Arm II: Participants are observed for 4 menstrual cycles. Participants in both arms undergo 24-hour collection of urine on days 7, 8, and 9 of menstrual cycles 1 and 6. F_2-isoprostanes are measured via gas chromatography-mass spectrometry (GC/MS). Estrogens are also measured by GC/MS and include E_1, E_2, E_3, 2-OHE_1, 2-OHE_2, 4-OHE_1, 4-OHE_2, 2-MeE_1, 2-MeE_2, 4-MeE_1, 4-MeE_2, and 16OHE_1.