Why Do Women Bleed Every Month?

Your period comes each month because something else didn’t happen.

From the onset of your first menses (menarche) until the cessation of your menses (menopause) the sole purpose of your monthly cycle is to reproduce. The bottom line, you bleed each month because you didn’t get pregnant.

What Controls When My Period Comes?

Believe it or not, your uterus is more of a bystander in this monthly process. The main players are two structures in the brain the hypothalamus and the pituitary gland along with the ovaries.

Technically this is referred to as the hypothalamic-pituitary-gonadal axis. When the interactions of this neuroendocrine trio function properly ovulation and (if pregnancy doesn’t result from fertilization of the released egg) menstruation happen at regular intervals.

How Does it Work?

As a result of a complicated and coordinated series of hormonal and structural changes the regular menstrual cycle occurs. The first day of menstrual bleeding is actually the first day of the next cycle. A regular menses or period occurs on average every 28 days or about 14 days after a regular ovulation.  If the complex hypothalamic-pituitary-ovarian axis does not function properly resulting in ovulation then your menstruation does not function properly.

A Deeper Look

In the next paragraphs, I will give a more detailed explanation of these events (if you are interested) to help you understand how the normal cycle works.

Let's think about the menstrual cycle in two parts or phases with the days in parenthesis representing an average 28-day cycle.

Phase 1 (Day 1-14)

As your period begins and the built up lining from the previous cycle is shedding,the hypothalamus produces gonadotrophic releasing hormone (GnRH) that stimulates the anterior pituitary gland to release follicle stimulating hormone (FSH).

FSH then stimulates the ovaries to prepare an egg for ovulation. FSH causes the follicles of the ovaries to produce estrogen and this process begins during menstruation. Estrogen levels rise quickly and turn down the production of FSH.  Eventually, one follicle on one of your ovaries will grow more rapidly than the rest and will become the “dominant follicle”. The dominant follicle will then ultimately release the egg at ovulation.

This first phase of the cycle, the part before ovulation, is called the follicular phase from the perspective of the ovary. Under the influence of the rising estrogen levels, the lining of the uterus or endometrium begins to thicken or proliferate. So, from the perspective of the uterus, the first part of your cycle is called the proliferative phase.

Ovulation (Day 14)

Ovulation occurs after a surge in another anterior pituitary hormone called luteinizing hormone (LH).  In response to this spike in LH, the dominant follicle releases the egg (oocyte)  and ovulation occurs.

This usually happens on cycle day 14. 

Phase 2 (Day 14-28)

Now the follicle that released the egg begins to shrink. It continues to produce estrogen but now also begins to produce progesterone and is now called the corpus luteum.

This second part of your menstrual cycle is called the luteal phase from the perspective of the ovary. Although both estrogen and progesterone are produced during this part of the cycle progesterone concentrations dominate.

Under the influence of progesterone, the lining of the uterus begins to change in ways to prepare it for implantation of a fertilized ovum. Remember that the biological purpose of this monthly cycle is to prepare for pregnancy. In the first half of the cycle, the lining began to build up now the lining becomes thicker and more complex with glands and blood vessels and tissue swelling.  These are all changes that get the uterine lining ready for the process of implantation and pregnancy. When naming this phase from the perspective of the uterus, it is called the secretory phase.

If implantation of a fertilized egg does not occur, the corpus luteum in the ovary continues to shrink away and estrogen and progesterone levels continue to fall. When this happens, the blood vessels that expanded in the thickened lining constrict and cut off blood flow. The thickened lining now without blood flow to support it, dies and is shed from the uterus. This cyclic shedding of the lining of the uterus because implantation did not occur is your menstruation and you are back to day 1 of a new cycle.


ACOG Practice Bulletin Number 136 July 2013

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