The Menstrual Cycle

What is menstruation?

Menstruation is one part of a woman’s menstrual cycle which includes the shedding of the endometrium (lining of the uterus) that occurs throughout a woman’s reproductive life. With each monthly (on average) menstrual cycle, the endometrium prepares itself to nourish a fetus, as increased levels of estrogen and progesterone help to thicken its walls. If fertilization does not occur, the endometrium, coupled with blood and mucus from the vagina and cervix (the lower, narrow part of the uterus located between the bladder and the rectum) make up the menstrual flow (also called menses) that leaves the body through the vagina.

When does menstruation begin?

On average, menarche (a young woman’s first menstrual period) occurs between the ages of 12 and 14 years old – generally two years after her breast budding (average age 10 to 12 years old), and, in most cases, not long after the onset of pubic hair (average age 12 years old) and underarm hair. Stress, various types of strenuous exercise, and diet can affect the onset of menstruation and the regularity of the menstrual cycle.

The American College of Obstetricians and Gynecologists recommends that a young woman consult her physician if she has not started to menstruate by the age of 16, and/or if she has not begun to develop breast buds, pubic hair, or underarm hair by the age of 13 or 14.

What is ovulation?

When a young woman reaches puberty, she begins to ovulate – a process in which a mature egg cell (also called an ovum), ready for fertilization by a sperm cell, is released from one of the ovaries (two female reproductive organs located in the pelvis). If the egg is fertilized by a sperm cell as it travels down the fallopian tube, then pregnancy occurs and it becomes attached to the lining of the uterus until the placenta (an organ, shaped like a flat cake, that only grows during pregnancy and provides a metabolic interchange between the fetus and mother) develops. If the egg does not become fertilized as it travels down the fallopian tube on its way to the uterus, the endometrium (lining of the uterus) is shed and passes through the vagina (the passageway through which fluid passes out of the body during menstrual periods; also called the birth canal), a process called menstruation.

As the average menstrual cycle lasts 28 days (starting with the first day of one period and ending with the first day of the next menstrual period), most women ovulate on day 14. At this time, some women experience minor discomfort in their lower abdomen, spotting, or bleeding, while others do not experience any symptoms at all.

A woman is generally most fertile (able to become pregnant) a few days before, during, and after ovulation.
How long is a menstrual cycle?

For menstruating women, an average menstrual cycle lasts 28 days – starting with the first day of the last period (which, on average, lasts six days, with some women having a very light flow and others having a very heavy flow) and ending with the first day of the next menstrual period. However, the length of women’s cycles varies, particularly for the first one to two years after menarche (a young woman’s first menstrual period). Women may have cycles as short as 23 days, or as long as 35 days. However, anything that deviates from this range is considered abnormal and may require medical attention.

What happens during the menstrual cycle?

Females of reproductive age (anywhere from 11-16 years) experience cycles of hormonal activity that repeat at about one-month intervals. (Menstru means “monthly”; hence the term menstrual cycle.) With every cycle, a woman’s body prepares for a potential pregnancy, whether or not that is the woman’s intention. The term menstruation refers to the periodic shedding of the uterine lining.

The average menstrual cycle takes about 28 days and occurs in phases: the follicular phase, the ovulatory phase (ovulation), and the luteal phase.

There are four major hormones (chemicals that stimulate or regulate the activity of cells or organs) involved in the menstrual cycle: follicle-stimulating hormone, luteinizing hormone, estrogen, and progesterone.

Follicular phase
This phase starts on the first day of your period. During the follicular phase of the menstrual cycle, the following events occur:

-Two hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH) are released from the brain and travel in the blood to the ovaries.
-The hormones stimulate the growth of about 15-20 eggs in the ovaries each in its own “shell,” called a follicle.
-These hormones (FSH and LH) also trigger an increase in the production of the female hormone estrogen.
-As estrogen levels rise, like a switch, it turns off the production of follicle-stimulating hormone. This careful balance of hormones allows the body to limit the number of follicles that complete maturation, or growth.
-As the follicular phase progresses, one follicle in one ovary becomes dominant and continues to mature. This dominant follicle suppresses all of the other follicles in the group. As a result, they stop growing and die. The dominant follicle continues to produce estrogen.

Ovulatory phase
The ovulatory phase, or ovulation, starts about 14 days after the follicular phase started. The ovulatory phase is the midpoint of the menstrual cycle, with the next menstrual period starting about 2 weeks later. During this phase, the following events occur:

-The rise in estrogen from the dominant follicle triggers a surge in the amount of luteinizing hormone that is produced by the brain.
-This causes the dominant follicle to release its egg from the ovary.
-As the egg is released (a process called ovulation) it is captured by finger-like projections on the end of the fallopian tubes (fimbriae). The fimbriae sweep the egg into the tube.
-Also during this phase, there is an increase in the amount and thickness of mucus produced by the cervix (lower part of the uterus.) If a woman were to have intercourse during this time, the thick mucus captures the man’s sperm, nourishes it, and helps it to move towards the egg for fertilization.

Luteal phase

The luteal phase begins right after ovulation and involves the following processes:

Once it releases its egg, the empty follicle develops into a new structure called the corpus luteum.
The corpus luteum secretes the hormones estrogen and progesterone. Progesterone prepares the uterus for a fertilized egg to implant.
-If intercourse has taken place and a man’s sperm has fertilized the egg (a process called conception), the fertilized egg (embryo) will travel through the fallopian tube to implant in the uterus. -The woman is now considered pregnant.
-If the egg is not fertilized, it passes through the uterus. Not needed to support a pregnancy, the lining of the uterus breaks down and sheds, and the next menstrual period begins.

How many eggs does a woman have?

During fetal life, there are about 6 million to 7 million eggs. From this time, no new eggs are produced.

The vast majority of the eggs within the ovaries steadily die, until they are depleted at menopause. At birth, there are approximately 1 million eggs; and by the time of puberty, only about 300,000 remain. Of these, 300 to 400 will be ovulated during a woman’s reproductive lifetime. The eggs continue to degenerate during pregnancy, with the use of birth control pills, and in the presence or absence of regular menstrual cycles.