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11.1.5. The menstrual cycle

The menstrual cycle

The menstrual cycle is the set of periodic changes that occur in the female reproductive system in order to prepare the body for fertilization of the ovum and pregnancy. It begins in women from 10 to 14 years, with menarche, and disappears between 45 and 55 years, with menopause.

The menstrual cycle usually has an average duration of 28 days, although it can vary. Only a single egg usually matures in each cycle.

The menstrual cycle consists of two phases:

  • Follicular phase. A group of follicles develop and a single follicle matures and increases in size until ovulation occurs, when the follicle ruptures and the mature egg is released. It occurs from day 1 (the first day of menstruation) to day 14 of the cycle .
    • The pituitary gland secretes follicle-stimulating hormone (FSH), which stimulates the growth of follicles, although only one will mature.
    • In the developing follicles, its cells begin to secrete estrogen, hormone that causes the increase of the endometrium of the uterus , which becomes thicker and filled with blood vessels.
    • When the level of estrogens increases to its maximum, the pituitary gland secretes luteinizing hormone (LH), causing ovulation or expulsion of the ovum from the follicle, approximately in the middle of the menstrual cycle.
    • The egg travels through the fallopian tubes to the uterus.
  • Luteal phase. It lasts from day 15 to 28 of the cycle, from ovulation until menstruation begins.
    • After ovulation, the remnants of the follicle form the corpus luteum or yellow, which will last until pregnancy or menstruation occurs.
    • The luteinizing hormone (LH) causes the corpus luteum begins to secrete progesterone, the hormone that prepares the endometrium for embryo by receiving it acquires its maximum thickness.
    • If fertilization does not occur, about 14 days after ovulation, the production of progesterone will decrease, causing the lining of the uterus, the endometrium, to shed and go out through the vagina, causing a bleeding called menstruation or rule, which It lasts for about four days and will start another new menstrual cycle.
    • If the egg meets a sperm in the fallopian tube, fertilization can occur and the embryo implants in the uterus, estrogen and progesterone will continue to be produced to prevent the endometrium from shedding, until the placenta is created.

The egg , after ovulation , is released and travels to the uterus. If you don't find a sperm within 24 hours, the egg dies. This occurs almost always, either because there has been no copulation or because the sperm has not found the egg due to the use of a contraceptive method or for other reasons.

The life of sperm is longer, about five days, so the days before ovulation and the day of ovulation are the most fertile days of the menstrual cycle and when there is the greatest chance of pregnancy.


Curiosity: Do all animals have menstruation?

The period, in addition to humans, appears in some primates (Catarrhini, our closest relatives), in some species of bats and Elephant shrews.

Almost all mammalian species have a reproductive cycle or estrous cycle , in which changes occur in the ovary as a result of hormonal changes that regulate the receptivity of the female. During this cycle, the uterine lining grows and, if there is no fertilization, it returns to its initial state without any bleeding. Unlike the human menstrual cycle, the estrous cycleIt is usually regulated by other factors, such as seasonal changes, temperature, food availability, male odors, etc. Mares, sheep, cats, goats, for example, only have one cycle in one season of the year. In some cases there is a certain periodicity, as in dogs (diesel, two estrus per year). Other species, on the other hand, such as the cow or the sow, have estrous cycles throughout the year.

You may have heard that bitches have periods, but that bleeding is not period. It occurs when the heat begins because it increases the concentration of estrogens that favor the dilation of blood vessels so that more blood reaches the female genital tract and passes through the walls of the vagina.