Development of the Menstrual Cycle

Sexual development in puberty begins with activation of the gonadotropinreleasing hormone (GnRH) pulse generator and the hypothalamic-pituitary system. GnRH secretion in the hypothalamic centers promotes the secretion of gonadotropins (FSH and LH) in the anterior pituitary gland. LH secretion increases steeply with an increase of LH pulses. This stimulates ovarian function and estradiol production in the follicles (fig. 1). Estradiol promotes effects on the target organs, particularly proliferation of the endometrium.

When the endometrium becomes too high for the prevailing estradiol level, its growth is interrupted and estrogen breakthrough bleeding occurs. This is the first menstruation, the menarche (fig. 1).

The age of menarche varies considerably with the epidemiological situation, and environmental, social and personal factors. In our Central European region, menarche usually occurs between 11 and 13 years of age. At this time the sexual development of the breasts and pubic hair is at about Tanner stage B3, P3 to B3, P4. The beginning of sexual development can usually be recognized about 2 years before menarche. These correlations are important for distinguishing between normal and disturbed development. After menarche, estradiol secretion increases and sexual development continues to the adult form, Tanner stage B5, P5, with further development of the reproductive organs.

The menstrual cycle after menarche is usually anovulatory with more or less irregular intervals. The quantity of bleeding varies. With increasing estradiol and with beginning and normalizing of the pulsatile secretion of GnRH the cycles gradually become ovulatory. During puberty, pulsatile gonadotropin secretion extends from the night hours into the daytime until the pulse frequency and amplitude reach those of ovulatory menstrual cycles. The establishment of a regular ovulatory cycle takes a rather long time, about 1 - 3 years after menarche. After the period of estrogen breakthrough bleeding without secretory conversion of the endometrium, ovulation occurs, initially irregular with a deficient luteal phase. The secretion of progesterone stabilizes incrementally, again with individual differences. The end result of this continuous development from anovulation, corpus luteum insufficiency and oligomenorrhea is a biphasic ovulatory cycle with normal corpus luteum function. At this time the hormone levels are those of normal ovulatory cycles (table 1); GnRH is secreted in pulses, the pulse generator is stabilized, and the regulation systems induce appropriate negative and positive feedback mechanisms. This adult form of the cycle is the basis for normal reproduction. The steps of this developmental process can be reversed in girls with secondary menstrual disturbances after a period of normal cycles ().