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Menstrual Cycle

The Physiological Phenomenon for which the Human Race Exists as it does Today

Menstrual cycle

The physiological phenomenon in which the unused endometrium is shed and voided from the body as the menses or period or menstrual flow.
This term was derived in the context of human females who happen to cycle in about the same length of time as a lunar month, it is also applied to other species whose cycles are not one month long.

Human females are somewhat unusual

Females of most species are only receptive around the time of ovulation (release of a fertile egg)
A human female is more receptive around the time of ovulation
But that is not the only time she is receptive
Human females generally are receptive to sexual activity throughout their cycles

The average menstrual cycle in humans ranges about 20 to 40 days in length, with a statistical average of about 28 to 29 days.

By convention, the first day of a woman’s period is considered to be day 1 of her cycle.

The first 3 to 7 days are generally the menstrual flow phase and during this time, all hormones involved are at low levels.

Five hormones involved in controlling the female cycle

Gonadotropin releasing hormone (GnRH) secreted by the hypothalamus
Follicle-stimulating hormone (FSH) &
Lutenizing hormone (LH) secreted by the pituitary gland
Estrogen &
Progesterone secreted by ovaries

The first half of a woman’s cycle is the proliferative phase (follicular phase), during which the endometrium starts to thicken.

The pituitary secretes FSH which causes (usually one) follicle to mature and the ovaries to secrete estrogen.
The ovarian estrogen secretion gradually increases until just prior to ovulation.
This gradually supresses secretion of FSH and stimulates the hypothalamus to secrete a larger amount of GnRH which, in turn, triggers the pituitary to secrete a burst of LH, causing ovulation.

During the proliferative phase, a woman’s body temperature is low.

Sometimes there is a slight rise near the end of the phase during the pre-ovulatory burst of LH before it dips again at ovulation.

Throughout this phase, the cervical mucus becomes progressively clearer and thinner


Day 14 of an average 28-day cycle
In response to the surge of LH the rupture of the follicle and release of the egg
LH stimulates the remaining follicle cells to form a corpus luteum after ovulation
Sharp drop in the woman’s body temperature
cervical mucus becomes very thin and clear and forms “threads”