Summary: One hundred and seventy-five women took part in a comparative clinical trial of four progestogen-ouly oral contraceptives and were followed for either a year or until treatment was discontinued. Megestrol acetate 0-25 mg. was found to be a very ineffective contraceptive, 21 out of 43 women becoming pregnant. One, three, and four pregnancies occurred during treatment with norethisterone acetate 0'3 mg., norgestrel 0.05 mg., and chodinone 0-5 mg., respectively, corresponding to pregnancy rates of 4, 9, and 12 per 100 woman-years of use.All three effective progestogens were very much less acceptable than modern low-dose combined oral contraceptives. Discontinuation of treatment for medical reasons (particularly menstrual disturbances) during the course of only one year affected 24% receiving norethisterone acetate, 38% receiving norgestrel, and 46% receiving chlormiinone.
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