The female genital system undergoes in women, from puberty (between 8 and 12 years) to menopause (between 45 and 50 years) regular cyclical modifications, the menstrual cycle, which lasts an average of 28 days, numbered starting from the first day of the menstrual flow , and which involves, through the action of hormones, important modifications of some target structures.
Menstrual Phase (Days 1 – 5)
The most superficial layer of the uterus , called the endometrium, undergoes necrosis (cell death) and detaches in flaps, leaving the veins and arteries that flow through it exposed. Therefore there is a loss of about 40 milliliters of blood mixed with cellular necrotic residues of the endometrium. At the same time, the growth of some follicles begins in the ovary , while the secretion of FSH , or follicle-stimulating hormone , produced by the pituitary gland increases, which in women determines the maturation of the follicle and stimulates the production of estrogen by the ovary.
Proliferative Phase (Days 6 – 14)
In the uterus, the superficial circulation of the endometrium is restored and the layer of epithelial cells lining the vagina thickens to create an environment conducive to sperm .
In the ovary, only one follicle continues to mature, while the others undergo involution.
Estrogens increase significantly until they reach a maximum peak just before ovulation and then decrease rapidly. Progesterone, another hormone secreted by the ovary, rises slowly. FSH decreases and then rises again just before ovulation while there is a large increase in LH, or Lutenizing hormone, always produced by the pituitary gland and which collaborates with FSH in maintaining the normal production of estrogen by the ovary as well as determining the formation of the corpus luteum after ovulation and stimulating the production of progesterone.
Ovulatory Phase (Day 14-15)
In the uterus, the endometrium reaches its maximum thickness; in the ovary there is the rupture of the follicle and the expulsion of the egg contained in it; estrogen decreases rapidly and progesterone continues to increase. The basal body temperature , best assessed in the morning as soon as you wake up with a special thermometer to be inserted into the vagina, rises by about half a degree.
Basal temperature measurement methods are based on this measurement, which some women use to establish the exact moment of ovulation. It must be remembered that this method cannot absolutely be effective for contraceptive purposes , due to the fact that most women do not have regular cycles, but rather it can be very useful when you want to plan a pregnancy . Therefore, it is by no means proven that all women, three days after the slight rise in temperature, can be considered no longer fertile.
Initial Secretory Phase (Days 16 – 23)
In the uterus, the endometrial epithelium remains at its maximum thickness; in the vagina, on the other hand, the epithelium thins; in the ovary, the corpus luteum is formed; estrogen and progesterone increase, while FSH and LH decrease.
Late Secretory Phase (24 – 28 Days)
In the ovary, the regression of the corpus luteum begins; estrogen and progesterone decrease while FSH and LH remain at low levels.
The reduction in progesterone levels leads to the flaking of the uterine lining (endometrium), then to menstruation; at this point the menstrual cycle can start again.