A report of the correlation between the pregnancy rates of low estrogen formulations and pill-taking habits of females studied.

The purpose of the investigation reported was to determine the lowest dose of steroid which would still maintain an acceptable contraceptive efficacy. 3 formulations of progestin: estrogen ratio of 50:1 and 3 with the ratio of 20:1 were studied (1.5/30 1/20 .5/10 .6/30 .4/20 and .2 mg norethindrone acetate/10 mcg ethinyl estradiol respectively). Total subjects were 1192 1393 178 1872 313 and 276 respectively; for the 6 formulations. Results showed that there is a large built-in margin of error for pills which decreases as the total dose decreases. It was concluded that adequate contraceptives protection can be achieved by formulations containing 10 mcg of ethinyl estradiol in women who are sufficiently motivated to take medication every day as directed.