Plasma levels of oxytocin during the menstrual cycle, pregnancy and following treatment with HMG.

In 15 women investigated during the follicular, ovulatory and luteal phases of the menstrual cycle, only small variations in plasma levels of oxytocin were found. There was no mid-cycle surge in oxytocin levels. A parallel variation in oxytocin and oestradiol levels was found in five of the women. In 10 women examined every 4th week during gestation starting at week 12, a small but significant increase in oxytocin levels was found. Pronounced fluctuations in oxytocin levels were observed in four of the pregnant women. These fluctuations were independent of the stage of gestation and were also observed in three of the menstruating women. Seven out of 12 women treated with human menopausal gonadotrophin exhibited a marked elevation of oestradiol levels; oxytocin levels also increased and were significantly elevated when the oestradiol level exceeded 2 pmol/ml. Oxytocin levels were higher in the pregnant and human menopausal gonadotrophin-treated women than in the menstruating women. The oxytocin levels appeared to be only partly related to the oestradiol level. It is possible that the stimulatory effect of oestradiol is antagonized by a concomitantly high progesterone level. Alternatively, there may not be a linear dose-effect relationship between oestradiol and the release of oxytocin.