The menstrual cycle is regulated by the hypothalamus, which stimulates the pituitary gland (a gland located at the base of the brain, immediately beneath the hypothalamus). The pituitary gland releases two hormones that regulate the female reproductive cycle, luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
LH and FSH influence the production of estrogens and progesterone, which in turn control the cyclical changes that occur in the lining of the uterus, including menstruation.
The menstrual cycle consists of 3 different phases and lasts for, usually, 28 days. However, cycles lasting between 21 and 35 days are considered normal. The cycle begins with the first day of menstruation and ends the day before the next menstruation. The cycle consists of 3 phases:

Phase 1: Follicular phase (before egg release)

This phase begins with menstruation and lasts for 12 to 14 days. The levels of estrogen and progesterone are low, which results in the onset of menstruation. Menstruation, also called the period, is the peeling of the wall of the uterus (endometrium) when there is no fertilization. That is, when the egg is not fertilized by any spermatozoa, there is peeling of the endometrial tissue, which causes loss of blood flow, also known as menstrual flow. During this phase, follicle-stimulating hormone levels increase slightly and stimulate the development of several follicles in the ovaries, with each follicle containing one ovule. Subsequently, levels of follicle-stimulating hormones (FSH) decrease and only one follicle continues its development. This follicle produces estrogen.
At this stage it is normal to feel cramps in the abdominal area. Weakness, headaches, irritability are also common symptoms. Do not worry! It's physiological, it's natural.

Phase 2: Ovulation Phase

At this stage the release of the ovum by the ovary - Ovulation. The egg, after being released, descends through the fallopian tubes to the uterus. Ovulation occurs because at this stage there is an increase in LH and FSH levels. LH stimulates the release of the egg, which usually occurs 16 to 32 hours after the onset of increased hormone levels. The level of estrogen reaches its peak during the increase and the level of progesterone begins to increase. At this stage you feel more excited and there is an increase in libido. It is the most fertile period of woman.

Phase 3: Luteal Phase

After ovulation, you enter the luteal phase. It lasts for approximately 14 days (if fertilization does not occur) and ends shortly before the menstrual period. LH and FSH levels decrease. The ruptured follicle (after releasing the ovum) heals and forms the corpus luteum. This stage leads to increased levels of progesterone and it is this hormone that is responsible for preparing the uterus in case fertilization occurs and the embryo is implanted in the wall of the uterus (endometrium). That is, progesterone causes the endometrium to thicken (fluid and nutrients increase) in order to nourish the implanted embryo. The mucus in the cervix also increases and becomes thicker. Progesterone also causes the body temperature to rise slightly, which remains elevated until the onset of the menstrual period. During most of the luteal phase, the estrogen level is high. Estrogen also stimulates the endometrium to thicken.
Increased levels of estrogen and progesterone stimulate the increase of milk ducts in the sinuses, which leads to dilation of the same. As a result, the breasts can become more tender.
At this stage, PMS (Premenstrual Tension) symptoms may also occur. You may feel more irritable, sensitive and moody.
If the ovum is not fertilized or if the fertilized egg does not implant, the corpus luteum degenerates after 14 days. At this stage, estrogen and progesterone levels decrease and a new menstrual cycle starts.

What if the embryo is implanted?

In this case, cells around the developing embryo begin to produce a hormone called Human Chorionic Gonadotrophin (HCG). This hormone maintains the corpus luteum, which continues to produce progesterone until the developing fetus can produce its own hormones. Pregnancy tests are based on detecting an increase in the level of this hormone. If HCG is detected, the result of the pregnancy test is positive.