Fertility Basics: How Menstruation Works

One of the first steps in understanding fertility is becoming aware of the physical processes the take place within the female reproductive system. Any disruptions in a woman’s menstruation, such as a missed period or irregular periods, could indicate a sign or a larger fertility problem. In order to understand how certain infertility conditions can affect reproductive health, it is important to learn the basics about the menstrual cycle and exactly what is involved in menstruation.

The Reproductive Years

A woman’s reproductive years are considered to begin with the onset of menstruation, lasting until she enters menopause. However, a woman will develop all the egg follicles she will ever have before she is even born. Typically, a woman will have several millions of egg follicles stored in the ovaries at birth. However, by the time she begins her first menstrual cycle, the number of immature eggs will have been reduced to about 400,000.

On average, every 21 to 35 days, a woman’s body will prepare 20 eggs for maturation. However, only one of these eggs will dominate and be released from the fallopian tubes for ovulation.

The Menstrual Cycle: Hormones

Any disruption that affects hormonal balance can lead to problems with menstruation and female fertility. There are five hormones involved in the menstrual cycle:

Estrogen
There are actually two different types of estrogen hormones responsible for menstruation. Estradiol hormones are responsible for the thickening of the uterine wall as well as changing vaginal and cervical mucus to allow sperm to travel more easily.

Androgen is converted into estrogen by the ovaries and helps eliminate immature egg follicles that will not be released for ovulation.

Progesterone
Progesterone hormones are produced by the egg follicles that are released for maturation each month. These hormones are responsible for preparing the uterine lining for the implantation of an egg, and also prevent egg follicles from further development.

Gonadotropin Releasing Hormone (GnRH)
Gonadotropin releasing hormones control estrogen production in the body and are produced by the hypothalamus in the brain. Estrogen levels are significantly decreased at the end of the menstrual cycle, and GnRH are responsible for signaling their production. High progesterone levels inhibit GnRH production.

FSH and LH
Follicle stimulation hormones (FSH) are themselves stimulated by GnRH and are secreted by the pituitary gland. FSH are responsible for stimulating egg follicles and assisting in maturation. The presence of FSH also increases estradiol production.

Lutenizing hormones (LH) are also secreted by the pituitary gland and are regulated by GnRH production. LH actually works with the egg follicles in the production of androgen.

How The Menstrual Cycle Works

The First Days
The first day of your period is considered the first day of your menstrual cycle. During these first few days of menstruation, a woman’s estrogen hormone levels decrease. This signals the thickening of the uterine lining known as the endometrium. Cervical mucus changes its consistency to allow easier sperm movement.

Increased estrogen levels also signal the production of follicle stimulating hormones (FSH), which signals the maturation of egg follicles. Only one out of twenty eggs will dominate and attract these hormones. FSH helps this egg follicle produce estradiol, which actually signals a decrease in the production of FSH in the pituitary gland. This process is what causes the excess follicles to die since these require FSH in order to continue to grow.

Ovulation
Estrogen levels continue to rise in the body following these first days of menstruation, and reach their peak about a day and a half (36 hours) prior to ovulation. This climax will cause a surge in the production of lutenizing hormones (LH).

The maturing follicle then releases an egg, which is collected by finger-like structures found at the end of the fallopian tubes (fimbraie) and begins to travel towards the uterus. This is what is known as ovulation. The follicle then begins to accumulate fatty substances and become the "corpus luteum." This signals the production of progesterone. Increasing progesterone levels prepare the endometrium for implantation of a fertilized egg.

Once an egg has been released into the uterus, it has about 72 hours to be fertilized. If the egg is fertilized, chorionic gonadotropin (hCG) hormones, also known as the pregnancy hormone, will be produced.

Menstrual Flow
If the egg is not fertilized, the lining of the uterus will begin to shed itself, and hormone levels will begin to decline. This will result in the menstrual flow. The drop in hormone levels signals the production of GnRH, which signals the beginning of another menstrual cycle.