The luteal phase of the cycle begins, which depends on the corpus luteum to produce progesterone. Its role is to finally prepare the endometrium to receive the embryo. Progesterone is secreted in large quantities from day 15 to 26 of the cycle. If fertilisation does not occur during ovulation, between the 26th and 28th day of the cycle, the corpus luteum disappears and progesterone production declines rapidly, resulting in a disruption of the blood supply to the endometrium and subsequent separation of the superficial layer of the endometrium, which manifests itself in subsequent menstrual bleeding. The start of a new menstrual cycle is counted from the first day of the onset of bleeding.

The reasons for the absence (permanent or periodic) of menstruation can be:

• physiological, e.g. pre puberty, pregnancy, postpartum
• developmental, e.g. underdevelopment of the ovaries,
• and also related to endocrine disorders,
• diseases of the reproductive organs,
• stress, etc.

Cycle disorders

There may also be disorders associated with the intensity, length and frequency of bleeding. Bleeding occurring fairly regularly, every 21 to 35 days, lasting 3-8 days, is considered normal. The average bleeding length is 5 days and the cycle length is 28-32 days. Only 15% of women have 28-day menstrual cycles. There is a close temporal relationship between ovulation and the menstruation that follows it. Menstruation only occurs when the ovum released from the ovary during ovulation has not been fertilised. This is when the changes that prepared the woman's reproductive organ for conception and the subsequent development of the pregnancy are muted. The hypertrophied and distended endometrium retracts, becomes necrotic, flakes off and is expelled to the outside with the blood from the ruptured vessels. This usually occurs between 12 and 16 days after ovulation.

Bleeding and pregnancy

It does happen, however, that despite becoming pregnant, menstruation-like bleeding, although usually much less heavy, continues for one or two cycles, a phenomenon that is quite rare. The length of menstrual cycles can vary. We speak of average cycles, when their length is about 28 days, short cycles or long cycles, when their length deviates from the above in one direction or the other. There are women whose cycle length is always the same or almost the same, and there are also women whose cycle length varies, which does not mean that the latter have to be treated and their cycles "regulated". Only in the case of a definitively established pathology do we treat the underlying disease and not the "length of the cycles". Slight cycle regularities often occur in perfectly healthy women, and the length of cycles is almost never perfectly equal, as many external factors can affect the length of the cycle (e.g. illness, severe stress, travel and related climate change and others).

Male fertility

The cyclical nature of fertility-related changes is hardly relevant to the man, whose body is capable of impregnating a woman almost continuously, from puberty to old age, and is not subject to the cyclical changes that are characteristic of female physiology. Spermatozoa are produced in the sperm-forming epithelium of the testes, which, through the tubules and network of the testis, make their way to the epididymis, where they can reside for an extended period of time. During intercourse, the spermatozoa enter the vas deferens and, during ejaculation, exit through the urethra. Semen consists of millions of sperm suspended in the secretions of the testicle, epididymides and prostate gland, which enter the fallopian tubes from the vagina via the cervix and uterus. The ability of the sperm to fertilise persists in the woman's body for up to 72 hours, or even longer. It follows that fertilisation can occur even if sexual intercourse took place three days before the onset of ovulation.