Menstruation is the monthly fluctuation of hormones that controls the female body’s production and release of an egg from the ovaries into the uterus for fertilization. It culminates with the discharge of the uterine lining if a fertilized egg fails to implant in the uterus. The hormones typically fluctuate on a twenty-eight-day cycle; however, each body has its own rhythm, so some people experience cycles that are longer or shorter. The menstrual cycle is a cooperative act between the ovaries, uterus, and the hormones that control them.

There are several hormones involved in the menstrual cycle, and they are all important to the overall process. Hormones act as regulators of communication between various parts of the body. In this case, these hormones communicate between the hypothalamus in the brain and the reproductive organs. Follicle-stimulating hormone (FSH) acts on different areas depending on the sex of a person. In females, follicle-stimulating hormone is responsible for promoting the growth of the egg cell. The next hormone, luteinizing hormone (LH), is responsible for triggering ovulation as well as maintaining the corpus luteum, which develops from the follicle after ovulation. In short, FSH matures the egg cell, and LH controls the ovulation process. Estrogen is the main female hormone and has many important uses throughout the body, both in men and women. In the menstrual cycle, estrogen is responsible for both the growth of the uterine lining and the regulation of FSH. Progesterone is also important during menstruation. It is secreted by the corpus luteum in the ovary in order to support blood flow to the endometrium. Progesterone’s role also varies in later stages of the menstrual cycle depending on if there is an embryo to maintain.

The ovarian process begins at the beginning of the menstrual cycle. There are two phases in this process: the follicular phase and the luteal phase. The follicular phase occurs first. This is when FSH stimulates the ovary to develop and mature an egg cell. This begins after the cessation of the previous menstrual flow (menses). Many cells begin to develop during this time; however, typically only one reaches full maturation as once a cell becomes dominant, the others regress. After an egg cell reaches its full maturation, there is a spike in LH, which causes the next step, ovulation. Ovulation is when the mature egg cell breaks from its casing, pushes out through the ovarian wall, and travels down the fallopian tube into the uterus. A person is most fertile during the few days before and after ovulation. This is because as the egg travels down the fallopian tube, it is possible for fertilization to occur if sperm cells are present.

Just after menses, while the ovaries are in the follicular stage, the uterus is in the proliferative phase. This is when estrogen is at its highest production. While glands in the uterine wall grow, the tissue that lines the inner wall of the uterus (endometrium) thickens. The endometrium is at its thickest when ovulation occurs. This timing is important because the thickened wall is where the fertilized egg will be implanted. Implantation is when the fertilized egg burrows into the endometrium and begins to grow. It is also possible to have an egg that is fertilized but does not implant.

The second half of the ovarian process is called the luteal phase. After the release of the egg out of the casing to the fallopian tubes, the casing remains in the ovaries and is known as the corpus luteum. LH promotes the extended life of the corpus luteum while it secretes progesterone. These levels peak in the week after ovulation. There are two possibilities after this point. If fertilization and implantation take place, the implanted embryo will produce its own hormones, causing the corpus luteum to grow and secrete even more progesterone to maintain the pregnancy. If fertilization and implantation do not occur, then the corpus luteum will disintegrate over the remaining time of the cycle.

If there is implantation and an embryo starts to form, the increased progesterone will help maintain the endometrium and increase blood flow, and the endometrium will continue to thicken in order to better maintain the embryo. If there is no pregnancy, the degeneration of the corpus luteum will cause progesterone levels to drop. When the hormone levels drop, the endometrium loses its blood supply and sheds itself during menses. Both the endometrium tissue and blood from the tissues detach and flow down the cervix and out of the vagina.