Bleeding patterns with low-dose, monophasic oral contraceptives

Cycle control over 12 months with low-dose oral contraceptives (OCs) was analyzed using calendars of bleeding on pill-taking days 1 through 21 (intermenstrual bleeding; IMB). One preparation contained 0.5 mg norethindrone and 0.035 mg ethinyl estradiol (NET + EE), the other 0.3 mg norgestrel and 0.03 mg ethinyl estradiol (Ng + EE). Half the subjects had previously used OCs containing greater than or equal to 0.05 mg estrogen (switch-over); the others had not previously used OCs for 2 months or more (fresh). Fresh subjects reported more IMB than switch-over subjects, especially during the first three cycles; IMB decreased over time for both groups. Ng + EE subjects had fewer IMB episodes during the early cycles than NET + EE subjects. Daily incidence of IMB formed a characteristic W-shaped curve in the NET + EE subjects that was most apparent in early cycles.