Norethindrone was given to 210 women as a contraceptive, at a dose of 10 mg. per day for 20 days of each menstrual cycle, in order to determine what maximal effectiveness could be achieved in a carefully designed field trial. In a total of 6,232 cycles there were no pregnancies and no rejections of the treatment due to side effects. Of 18 subjects who interrupted the use of norethindrone to plan additional pregnancies, 16 became pregnant within the first 3 cycles.
Goldzieher JW, Moses LE, Ellis LT. Study of Norethindrone in Contraception. JAMA. 1962;180(5):359–361. doi:10.1001/jama.1962.03050180005002
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