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In order to study the effect of the Lippes loop on myometrium, uterine activity was recorded by the intrauterine open-end catheter technique in 11 parous women. The loop did not influence the activity up to the fourth or fifth postovulatory day. After that day, a precocious development of “prelabor-like” activity was observed, coinciding with the transport and nidation of the ovum. This abnormal uterine activity may be one factor in the mechanism of effect of the loop.

Uterine activity was studied in 11 parous women with normal ovulatory menstrual cycles by means of the intrauterine “open-end” catheter technique. From day 6 to day 18 of 28 day cycles the same type of activity was found, i.e., frequent contractions (2 to 4 per minute) of low amplitude (3 to 10 mm. Hg) and short duration. After the eighth day before the next period, another type of contraction appeared with lower frequency (every 2 to 3 minutes), higher amplitude (around 25 mm. Hg), and longer duration (1 to 2 minutes). These contractions gradually increased and 4 days before the next period reached a “prelabor-like” pattern. A further increase to “laborlike” activity was observed around the onset of bleeding.

Intrauterine and myometrial pressure changes were measured in the intact puerperal human uterus (14 subjects). Major differences between the spontaneous myometrial activity of various regions were observed. Intramyometrial injection of procaine caused localized disruption of the recorded activity. This action of procaine was obliterated by concomitant oxytocin infusion.

The effect of withdrawal of estrogen and progesterone on in vivo human myometrial activity has been studied, with the use of the open-end catheter technique. When exogenous estrogen was given in reproductive years or in postmenopausal subjects, prelabor-like and labor-like activity were lacking upon withdrawal of therapy. The same applies to subjects with spontaneous anovulatory cycles. On the other hand, in the presence of progesterone as in normal ovulatory cycles or synthetic progestins administered as oral contraceptives, labor-like activity follows withdrawal. The postmenopausal subjects with atrophic vaginal smears have a contractility pattern with minimal activity which mimics the end of the menstrual phase in younger subjects.

The myometrial activity has been studied during cyclic and continuous treatment with contraceptive pills (norethisterone and ethinyl estradiol, Anovlar). On cyclic treatment the contractility pattern changes in the same way but more rapidly than in ovulatory cycles. On continuous treatment accentuation of the “prelabor-like” pattern is observed. Cessation of treatment is followed by “laborlike” activity. The relation between the two hormonal substances and the different activity patterns are discussed.