Understanding the Menstrual Cycle

The menstrual cycle includes all the mechanisms that prepare the body, each month, for a possible pregnancy. Two main hormones fluctuate during this cycle: estrogen and progesterone. These hormonal variations have an impact on human psychology and physiology. Indeed, hormones influence mood, morphology and also the appearance of the skin. They can even be closely linked to the appearance of more or less marked symptoms depending on the woman. For example, the luteal phase can cause premenstrual syndrome (PMS), which is irritability, breast tension, weight gain or 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 beginning of menstruation of the next cycle. The length of the cycle can vary from woman to woman and from month to month, but is generally between 23 and 35 days. These fluctuations in cycle length occur particularly during the pre-ovulatory phase (or "follicular phase"). The first half of the cycle is spent preparing for ovulation, while the second half prepares for the implantation of the egg or the start of a new cycle if fertilization has not occurred. The menstrual cycle can be divided into 3 key phases:

1. The menstrual phase: During this period, estrogen and progesterone levels are very low. The uterine lining (or endometrium), which has thickened during the previous cycle due to a supply of blood and nutrients to accommodate the fertilized egg, sheds and is evacuated in the form of more or less heavy bleeding. This stage typically lasts from 3 to 7 days and can be accompanied by pelvic or lumbar 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 of preparing the uterus to receive a fertilized egg begins again, with the ovaries producing new eggs. Indeed, inside each ovary there are thousands of follicles
   corresponding to cavities filled with liquid and where each follicle contains an immature egg. During each cycle, a number of these follicles develop simultaneously but only one will reach maturity. Indeed, following a competition between follicles, only the largest at this stage remains, it is then called "dominant follicle". All the others regress. This phenomenon takes place thanks to the production of follicle-stimulating hormone (F.S.H.) by the pituitary gland (a small gland located at the base of the brain), which stimulates the development of the follicles and the start of estrogen secretion as the follicle grows.
   While the ovaries are working to produce an egg, the lining of the uterus is being rebuilt and developed by estrogen. At this point in the cycle, the body has all the chances on its side to ensure the fertilization of the egg. This phase can be prolonged in the case of long cycles. Just before ovulation, estrogen levels continue to rise, resulting in the sudden release of a hormone called luteinizing hormone (LH). This peak of LH triggers the rupture of the dominant follicle and thus the expulsion of the mature ovum by one of the 2 ovaries into the fallopian tube: this is called "ovulation". It generally takes place on the 14th day but this can vary from one cycle to another. It is precisely this level of L.H. that allows us to predict ovulation within 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 toward the uterus. The egg lingers for 24 hours waiting to be fertilized. Following ovulation, the body's main concern is to maintain a possible pregnancy. After ovulation, the ruptured follicle turns into a corpus luteum and begins to produce a new hormone, progesterone. The purpose of progesterone is to further strengthen the uterine lining for implantation of the egg and thus prevent miscarriage in early pregnancy. It is at this stage of the cycle that you may experience symptoms of P.M.S. (breast pain, irritability, acne, etc.) During this time, the corpus luteum continues to produce progesterone but also begins to secrete estrogen. The high levels of these hormones block any further ovulation. If fertilization has occurred, the fertilized egg will lodge in the uterine lining. This implantation takes place about a week after fertilization. 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 estrogen and progesterone. It keeps the endometrium intact until the placenta is sufficiently mature to maintain the pregnancy. On the other hand, in the absence of fertilization, the corpus luteum atrophies and the levels of progesterone and estrogen decrease, which triggers menstruation and the beginning of a new cycle.