Comparison of serum and cervical mucus hormone levels during hormone-free interval of 24/4 vs. 21/7 combined oral contraceptives.

Background: This study analyzes levels of progesterone, estradiol, norethindrone (NET) and ethinyl estradiol (EE) in serum and levels of NET in cervical mucus on the last day of the hormone-free interval (HFI) in users of 24/4 [norethindrone acetate (NETA)/EE-24] vs. 21/7 (NETA/EE-21) regimens.

Study Design: This was a randomized controlled, crossover, equivalency trial.