The menstrual cycle is a physiological process that occurs in all fertile women and usually lasts 28 days. Controlled mainly by the hormones FSH and LH, the menstrual cycle is the period between the beginning of one and the beginning of the next menstruation. Some women have shorter periods, up to 21 days, and others have longer periods, up to 35 days.
There is also the irregular menstrual cycle, which is one in which it is not known when menstruation will come. It is most common during adolescence (especially in the first three years of menstruation), shortly after pregnancy and in the pre-menopause phase, due to hormonal changes in these phases.
Stages of the menstrual cycle
The menstrual cycle is a complex process that can be divided equally into two phases: follicular phase and luteal phase, involving the control of several hormones.
The follicular phase begins on the first day of menstruation (the first day of the cycle). At the beginning of this phase, the hormones estrogen and progesterone are low, the wall of the uterus (endometrium) is very thin and the ovary is at rest. This phase lasts an average of 12 days.
The pituitary gland (pituitary gland), located in the central nervous system, increases the production of the hormone called follicle stimulating hormone (FSH), which stimulates ovarian follicles. With FSH, follicles develop, grow and mature. The follicles begin to produce estrogen and, as estrogen levels rise, one of the follicles becomes dominant, with greater development than the others, which stop growing. This dominant follicle is responsible for releasing the egg at the time of ovulation.
Estrogen also acts on the uterus, preparing it for possible pregnancy: the endometrium (membrane of the wall of the uterus) acquires layers and becomes thicker.
The maximum estrogen concentration occurs one day before ovulation and, at that moment, the luteinizing hormone (LH) is released from the pituitary gland. This occurs in the middle of the cycle, which is equivalent to the 14th day in the case of a 28-day menstrual cycle.
At this stage, the woman begins to produce a viscous mucus, called fertile mucus, which favors the mobility of sperm. The release of the luteinizing hormone completes the maturation process of the dominant follicle and, after 36 hours of its release, the egg is released.
When the woman ovulates, the egg is released towards the tubes and only the corpus luteum (structure responsible for the production of estrogen and progesterone) remains in the ovary. The luteal phase prepares the uterus for the start of the next menstruation.
After the egg is released, it is viable for approximately 12 to 24 hours, which means that fertilization is more likely when there are already sperm present before ovulation.
When fertilization occurs, the placenta produces HCG (human chorionic gonadotropin), a hormone that prevents the occurrence of another ovulation, keeping the action of the corpus luteum constant.
When there is no fertilization, high concentrations of progesterone reduce FSH and LH secretions. With this, the corpus luteum regresses and decreases the concentrations of estrogen and progesterone, which causes menstruation.