Postabortal contraception with norethisterone enanthate injections.

Intramuscular injection of 200 mg norethisterone enanthate (NET-EN) was given to five women on the day of first trimester abortion. The injection was repeated twice at eight-week intervals, and thereafter at twelve-week intervals for one year. Plasma samples were collected for FSH, LH/hCG, estradiol, progesterone and norethisterone (NET) determinations. NET-EN did not affect the elimination of hCG, estradiol or progesterone. Plasma NET concentrations reached a peak (5.5-11 ng/ml) in about ten days after the injection and declined constantly thereafter, to levels of 0.18-0.64 ng/ml at 8 weeks after the injection. NET-EN postponed the increase in FSH secretion until 17-20 days after the injection, i.e. until plasma NET concentrations fell below 3 ng/ml. In three out of five women some follicular activity was present 5 weeks after NET-EN injection as evidenced by increased plasma estradiol concentrations. No ovulation occurred within 8 weeks after NET-EN injection, as judged by low progesterone values. There was a definite accumulation of NET during the first six months, when NET-EN injections were given at eight-week intervals. Mean plasma NET concentrations increased from 0.34 ng/ml at eight weeks to 0.78 ng/ml at 24 weeks. When the injection interval was increased to twelve weeks, the plasma NET concentrations prior to the next injection started to decrease. This was accompanied by increased follicular activity, culminating with one ovulation observed. It is concluded that in this population of women, an injection interval of less than twelve weeks is needed for ovulation inhibition.