Endometrial control: a comparative study of three oral contraceptives

Shortly after completing an OC cycle the relative hormonal deficit caused by discontinuing the hormones results in withdrawal bleeding. Similarly, if one or more oral contraceptive tablets are missed during a cycle, the resulting deficiency may cause breakthrough bleeding. A lower incidence of breakthrough bleeding on the day after one oral contraceptive tablet is missed and later onset of withdrawal bleeding indicate that a particular OC provides better endometrial control. These two measurements are used as indices to compare the endometrial control provided by Ovral, Norinyl 1/50, and Nortestrin 1 in a study of 480 women at the Planned Parenthood Clinic in Seattle during 1974 and 1975. An analysis of the data shows that the normal chance of breakthrough bleeding on a particular day when one oral contraceptive tablet is not missed on the previous day is 0.017, 0.022, and 0.022 for Ovral, Norinyl, Norlestrin users, but the chance on the day after a contraceptive tablet is missed increased by 13 to 16 times for each of the three OCs. Ovral provides more sustained endometrial control than Norinyl and Norlestrin, however, as indicated by its lower probability of breakthrough bleeding on the day a contraceptive tablet is missed and by its later onset of withdrawal bleeding.