Understanding the menstrual cycle.

The menstrual cycle encompasses all the mechanisms that prepare the body, each month, for a potential pregnancy. Two main hormones fluctuate during this cycle: theoestrogenand theprogesterone. These hormonal variations have an impact on human psychology and physiology. Indeed, hormones influence mood, body shape and also the appearance of the skin. They can even be closely linked to the appearance of symptoms that vary in intensity among women. For example, the luteal phase can trigger the premenstrual syndrome (P.M.S.), which can cause irritability, breast tenderness, weight gain or even hormonal acne, while the pre-ovulatory phase is the most asymptomatic.

The menstrual cycle begins on the first day of menstruation and ends at the start of the next menstrual period. Its duration can vary from one woman to another and from one month to the next, but it is generally between 23 and 35 days. These fluctuations in the cycle duration particularly occur during the pre-ovulatory phase (or "follicular phase"). The first half of the cycle is dedicated to preparing for ovulation, while the second half prepares for the implantation of the egg or the commencement of a new cycle if fertilisation has not occurred. The menstrual cycle can be divided into 3 key phases:

1. The Menstrual Phase: During this period, the levels of oestrogen and progesterone are very low. The uterine lining (or endometrium), which has thickened during the previous cycle due to an influx of blood and nutrients in preparation for a fertilised egg, sheds and is expelled in the form of bleeding of varying intensity. This stage typically lasts from 3 to 7 days and may be accompanied by pelvic or lower back pain, and menstrual migraines.

2. The follicular (pre-ovulatory) phase: During this stage, the body intensively prepares for ovulation. In the days following the start of menstruation, the process to prepare the uterus to receive a fertilised egg restarts, including the resumption of new egg production by the ovaries. Indeed, within each ovary are thousands of follicles, which are fluid-filled cavities, and each follicle contains an immature egg. In each cycle, a certain number of these follicles develop simultaneously, but only one will reach maturity. Indeed, following a competition between follicles, only the largest at this stage survives, it is then called the "dominant follicle". All the others regress. This phenomenon occurs thanks to the production of the follicle-stimulating hormone (F.S.H.) by the pituitary gland (a small gland located at the base of the brain) which will stimulate the development of the follicles and the start of oestrogen secretion as this follicle grows.

   While the ovaries are engaged in the production of an egg, the lining of the uterus rebuilds and develops under the influence of oestrogens. At this stage of the cycle, the body is optimising all chances to ensure the fertilisation of the egg. This phase can be extended in the case of long cycles. Just before ovulation, the level of oestrogens continues to rise, also triggering the sudden secretion of a so-called luteinising hormone (LH). This surge in LH causes the rupture of the dominant follicle and thus the expulsion of the mature egg from one of the two ovaries into the Fallopian tube: this is what we call "ovulation". It usually occurs on the 14th day but this can vary from one cycle to another. Indeed, it is this level of LH that allows the prediction of ovulation within the next 24 to 48 hours, and it is on this principle that ovulation tests are based.

3. The luteal (post-ovulatory) phase: Once the egg is released, it travels down the Fallopian tube towards the uterus. The egg persists for 24 hours, waiting to be fertilised. Following ovulation, the body's main concern is to maintain a potential pregnancy. After ovulation, the ruptured follicle transforms into the corpus luteum and begins to produce a new hormone, progesterone. Its purpose is to further strengthen the uterine lining for the implantation of the egg and thus prevent a miscarriage in early pregnancy. It is at this stage of the cycle that you may experience PMS symptoms (breast pain, irritability, acne, etc...) Meanwhile, the corpus luteum continues to produce progesterone but also begins to secrete oestrogen. The high levels of these hormones block any new ovulation. If fertilisation has occurred, the fertilised egg will lodge in the uterine lining. This implantation takes place about a week after fertilisation. Once the egg is implanted, a specific pregnancy hormone, human chorionic gonadotropin (H.C.G.), is produced. This hormone maintains the activity of the empty follicle, thus continuing the production of the hormones oestrogen and progesterone. It thus helps to keep the endometrium intact until the placenta is mature enough to sustain the pregnancy. However, in the absence of fertilisation, the corpus luteum atrophies and the levels of progesterone and oestrogen decrease, which triggers menstruation and thus the beginning of a new cycle.