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Biology of the menstrual cycle

  1. When a girl reaches puberty, she will start menstruating. A monthly discomfort, with bleeding often accompanied by pain. The externally noticeable bleeding is the end of the cycle that makes a woman fertile. Every month an egg develops and the uterus prepares for a possible pregnancy. If it is not fertilized, the already developed tissue is secreted. This cycle repeats until a woman enters menopause.

Menstrual cycle

  1. The female reproductive organs The cycle The ovarian cycle The uterus cycle Contraception through cycle manipulation Menopause Menstrual cycle abnormalities

The female reproductive organs

  1. Figure 1: The female genitalia (Fallopian tube = fallopian tube) / Source: CDC, Mysid, Wikimedia Commons (Public domain) Understanding the usefulness of menstruation requires an understanding of the reproductive system of the woman. The uterus (or uterus) is central, which is connected to the outside world through the vagina. A fetus can develop in the womb. Also connected to the uterus are the ovaries (or ovaries), through the fallopian tubes (oviduct). The ovaries contain the female sex cells: the eggs or oocytes. The stock of eggs is already created in the fetal stage; at birth, the ovaries contain about 700,000 follicles (egg-containing structures). A woman has to do with this supply all her life, no new eggs are added. This in contrast to the male sex cells (the spermatocytes or sperm cells)

The ovarian cycle

  1. The follicular phase The pituitary gland (part of the brain) releases hormones that affect the sex organs: FSH and LH. These hormones cause the follicles to grow and produce the hormone estrogen. Estrogen has a negative feedback on the pituitary gland: the release of FSH and LH is inhibited, which prevents multiple follicles from developing. Estrogen stimulates its own production by means of a positive feedback, so that the follicles that have already developed are stimulated to further development. At any stage of follicular development, some follicles undergo arthresia (hormone-regulated cell death) so that usually only one follicle eventually reaches final stage. The remaining follicle produces the hormones progesterone and inhibin in addition to estrogen. The latter has an inhibitory effect on the release of FSH. The concentrations of estrogen and progesterone reach a peak value. This increases the sensitivity of the pituitary gland to the hormone GnRH (produced by the hypothalamus, another part of the brain), causing it to produce a large amount of LH (called the LH surge). The LH surge enables the last step in oocyte development: cell division takes place, meiosis

The uterine cycle

  1. Proliferation phase Under the influence of the estrogen produced by the developing follicles, in the late follicular phase, a new layer is formed in the endometrium (the "lining" of the uterus). This is associated with an increased blood supply to the uterus. There is also mucus in the cervix (cervix)

Contraception through manipulation of the cycle

  1. The processes in the menstrual cycle serve as preparation for pregnancy. Various contraceptives


  1. By the age of 50, the ovaries lose the ability to respond to pituitary hormones, preventing the processes in the menstrual cycle from taking place. A woman then stops menstruating and is permanently sterile. We speak of menopause

Menstrual cycle abnormalities

  1. If a girl / woman of 16 years or older has never had a menstrual period, we speak of primary amenorrhea. Secondary amenorrhea

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