The menstrual cycle occurs in every woman from the age of about 12 years to about 52 years. The menstrual cycle is what prepares the body each month in anticipation of a pregnancy. Let us understand this significant process in some more detail.

The beginning of the menstrual cycle is counted from the first day of the menstrual bleeding.

The menstrual cycle consists of four main parts or phases:

1. Menstrual Phase

2. Proliferation or Follicular phase

3. Ovulatory Phase

4. Luteal Phase or Secretory Phase

1. Menstrual Phase: The menstrual bleeding occurs during this phase. This phase lasts for about 4 to 5 days on average. The maximum bleeding occurs mostly from day through day 3. But agains this varies from every woman to woman. In some women who have irregular periods, this phase might last for fewer or more days.

But why does menstrual bleeding occur? Well, let us understand the rest of the phases of the menstrual cycle to answer this question.

2. Proliferative Phase: During this phase, which lasts for about 12 to 14 days, the uterus continues to build a blood rich lining of tissue on its inner surface. While the uterus is at its work, the ovaries also help ripen a few of the ovarian follicles, out of the many that have been present since birth. There are several hormones that come into play and one of the ovarian follicles is allowed to grow while the rest stop growing.

3. Ovulatory Phase: Around day 14 of the menstrual cycle, the egg in the ovarian follicle fully develops and the ovary releases the egg into the fallopian tube. This is called ovulation. Normally, only one of the ovaries releases the egg. There is no particular rule that governs which of the ovaries should be releasing the egg. It may happen that both the ovaries release a egg and if both those eggs.

4. Luteal phase (or secretory phase): This is the last phase of the menstrual cycle. During the luteal phase, the egg awaits fertilization by a sperm for about 4 days. If the egg is fertilized during this time, it immediately closes its walls to prevent penetration by other sperms and starts dividing into more cells. At the same time, the cluster of cells keep moving towards the uterus. Once the cluster of cells reaches the uterus it implants itself on the blood rich lining of tissue, that was getting ready in anticipation of a fertilized egg.  When the implantation is successful, the woman is considered to be pregnant.

If during the luteal phase, the egg does not fertilized, it withers away and the uterus without receiving a fertilized egg, sheds away its blood rich lining which comes out of the vagina in the form of the menstrual bleeding during the menstrual phase. This starts the cycle all over once again.