Clinical and Laboratory Findings in a Trial of Norgestrel,a Low-dose Progestogen-only Contraceptive

Norgestrel, a progestogen-only oral contraceptive, was given continually at a dose of 75 μg/day to 144 women of proved fertility. It was an efficient contraceptive with a failure rate of 2·1% (assessed by the “life-table” method) within the first 12 cycles and 3·6% within the first 30 cycles (or 2·0 conceptions per 100 woman-years when assessed by the Pearl index). The overall conception rate for the entire trial period was 2·1% and 1·3 pregnancies per 100 woman-years respectively. Norgestrel caused a high proportion of irregular and generally short bleeding intervals, about one-fifth of the cycles lasting less than 17 days. This irregularity appeared to be due to individual variance in cycle length between women rather than that between their successive cycles. No confirmed instances of thromboembolism were observed. Norgestrel apparently exerts its contraceptive action by several mechanisms: reduction in the sperm penetrability of the cervical mucus and an impairment of luteal function appear important. The serum concentrations of cholesterol and globulin were significantly reduced in women taking norgestrel. Preliminary observations suggest that on discontinuing the medication fertility is promptly restored. Of the 144 women originally enrolled 57 (40%) withdrew for reasons connected with the method before completing 30 months on trial, over half of them because of the irregular menstrual pattern. Nonetheless, in view of its main clinical and laboratory characteristics and simple mode of administration, norgestrel appears to be a useful alternative to the combined type of pill for women unsuitable for, or unable to tolerate, oestrogen-containing preparations.