Edited by Eugenio Ciuccetti, Obstetrician
What is the menstrual cycle?
By menstrual cycle we mean the time interval in women that passes between the first day of a period and the first day of the next menstruation. This interval ideally lasts 28 days, but in reality even menstrual cycles of 24 or 32 days are still considered completely normal.
The interval between one menstrual period and the next increases to the two extremes of reproductive life, due to the greater frequency with which anovular cycles can occur in these periods.
Menses
That of actual menstruation - that is, the sign of non-fertilization - represents instead the moment of least fertility of the woman during this cycle. Menstruation lasts on average from two to six days, with a flow of about 30ml (although this is also very variable). However, after 35 years the duration of the flow tends to be reduced.
Menstruation is given by the exfoliation of the mucosa that covers the uterus internally: ie the endometrium. The structure of the uterus is in fact characterized by three layers: the perimeter (external coating), the myometrium (the thickest muscular layer), the endometrium (for the nesting and nutrition of the possible fertilized egg).
Endometrium which is conditioned - in its characteristics - by hormones such as estrogens and progesterone. These are produced by the ovary during the menstrual cycle. At the end of the cycle, their production drops sharply and - if there is no implant - menstruation is in fact.
Duration in the menstrual cycle
The menstrual cycle is divided into various phases. The first day is when menstruation is seen for the first time (sometimes preceded by slight bleeding not yet menstruating). On the 14th day we have instead ovulation, that is the culminating hormonal moment. Finally, after the 28th day, he redeems the 1st day of the next cycle.
As noted above, however, not all menstrual cycles last exactly 28 days. It is therefore important to emphasize that the phase preceding ovulation, the so-called follicular phase (proliferative and estrogenic) may vary; on the contrary, the days that separate ovulation from the subsequent menstruation are always and in any case 14. This second phase of the cycle is defined as luteinic (secretory and progestin).
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Follicular or proliferative phase
Of course with menstruation only the upper and functional part of the endometrium is expelled. Instead the basal part remains that will allow a new proliferation. Estrogens are responsible for this proliferative phase. The phase in which the endometrium grows. After ovulation, the endometrium, now reconstructed, enters a phase of secretory maturation (regulated primarily by progesterone). This transformation is aimed precisely at the eventual implantation of the embryo.
In the first half of the cycle - that is, from the last menstrual period to ovulation - not only is the reconstruction of the endometrium but also the growth of the so-called follicle oophorus at the level of the ovary. Indeed it is precisely the follicle that produces the estrogens that stimulate the proliferation of the new endometrium. This is why this phase is called - in addition to estrogen and proliferation - also follicular.
Ovulatory phase
Each follicle contains an oocyte. With ovulation the follicle reaches full maturity, until it explodes. At this point the oocyte can theoretically be fertilized.
Fertile Period
The fertile period, therefore, that in which conception can occur, revolves around the time of ovulation.
Identifiable moment by the same woman through the detection of some characteristic signs such as those related to the variation of the basal temperature or of the cervical mucus.
Before doing any calculations, however, those looking for a pregnancy, or those who absolutely want to avoid it, should keep in mind the following: meanwhile, as mentioned above, not all menstrual cycles have exactly the same duration and to vary is just the first phase, the one that goes from the last menstruation to ovulation; secondly, spermatozoa can survive in the cervix for up to 3-4 days after intercourse; and again, the egg has a maximum life of 12-24 hours.
Luteinic phase
After ovulation, the exploded follicle deflates, collapses, and turns into a corpus luteum. This now mainly produces progesterone but will survive only 15 days, unless a pregnancy is established. In this case it will continue to act for three months.
For all these reasons, in the absence of pregnancy, the phase that separates ovulation from subsequent menstruation is therefore called progestin, luteinic and secretory phase. Lutein because the follicle turns into a corpus luteum. Progestin because, unlike the follicle that produced only estrogens, the corpus luteum also produces progesterone. Secretive because now the endometrium, rather than proliferating, turns into a possible nesting.
Hypothalamus-pituitary-ovary axis
Upstream of this complex mechanism we have the so-called hypothalamus-pituitary-ovary axis. And it is precisely this sophisticated neuro-endocrine system that comes into action with puberty, leading to the appearance of the first menstruation, called menarche. The same system will then regulate each woman's menstrual cycle every month, up to the menopause.
The first element of this chain is the Hypothalamus. A structure located at the base of the brain and an integral part of the central nervous system. In a nutshell, we can say that its function - in this context - is to transform nerve and electrical impulses from the brain into hormonal signals that can be understood by the rest of the system. In particular, the hypothalamus releases a fundamental neurosecret called GNRH. This reaches the Adenohypophysis, ie the anterior part of the pituitary gland - that is, the conductor of our entire endocrine system - by modulating the secretion of LH and FSH. That is luteinizing hormone and follicle-stimulating hormone.
In fact, the latter act on the ovary - the female gonad, the last station of this chain - favoring the maturation of the follicle, the simultaneous production of estrogen and of course the same ovulation, from which the corpus luteum and progesterone. The rupture of the mature follicle takes place particularly within 24 hours of the so-called LH peak. Peak that occurs precisely following the maturation of the follicle and the consequent increase in the estradiol secreted by it.
The hormonal levels reached by estrogens and progesterone, in turn, affect the entire upstream cascade by stimulating or inhibiting the pituitary gland to release more FSH and LH.
Age and Fertility
It should also be remembered that the number of oocytes decreases over the years. While a newborn has about 2 million eggs available, a girl in puberty (that is, at the beginning of her reproductive life) will have just 400, 000. After that, over the years there will be a total of 300 to 400 ovulations, while all the other oocytes will undergo degeneration until complete exhaustion and then menopause; that is, at the end of menstruation and above all of one's fertile life.
NOTES: the term amenorrhea means the absence of menstruation while that of dysmenorrhea means the presence of painful menstruation especially in the pelvic and abdominal region.
