The menstrual cycle phases

The menstrual cycle phases

Each month, your reproductive system repeats a regular pattern of events (your cycle, or your menstrual cycle), all controlled by hormones.
The menstrual cycle is defined as the time from the first day of a woman’s period to the first day of her next period. Here are the basics of the menstrual cycle phases.

Normal menstrual cycle length

This may come as a surprise, but your menstrual cycle does not necessarily take place once a month. The average menstrual cycle time for women is 28 days, but your menstrual cycle may last from 21 to 35 days and still be normal.

If you have a short menstrual cycle, you may have a period more often than once a month. However, if your menstrual cycle lasts longer, you’re one of the women who have fewer periods in a year.

The menstrual phase (menstruation)

The menstrual phase is a woman’s monthly bleeding, commonly referred to as your period. Day one of the menstrual phase is day one of your period and is the first day of your menstrual cycle. This menstrual blood (also known as menses) is shed from the lining of your uterus (known as the endometrium). Menstrual blood is shed from the uterus through the cervix, vagina and out through the vaginal opening. This fluid may be bright red, light pink or even brown. A period usually lasts about three to seven days. The normal amount of menstrual flow for your entire period is about a quarter of a cup.

The follicular phase

During this phase, the hormone estrogen causes the lining of the uterus to grow, or proliferate. This lining, called the endometrium starts to develop to receive a fertilized egg should you become pregnant. The increase of another hormone, called the follicle-stimulating hormone (FSH), in turn stimulates the growth of ovarian follicles. Each follicle contains an egg. By late in the follicular phase of the menstrual cycle, only a single follicle will remain active.

The lining of your uterus begins to thicken in response to this increase in estrogen. Estrogen levels rise dramatically during the days before ovulation and peak about one day before ovulation. The surge in estrogen triggers a spike in yet another hormone – the luteinizing hormone, or LH. Ovulation occurs as this increase in LH causes the follicle to rupture and release an egg.

The ovulation phase

Ovulation is what happens when a mature egg (ovum) is released from your ovarian follicle to the nearest fallopian tube during your menstrual cycle. Sometimes two of these eggs can mature in a month.

The increase in LH triggers ovulation. The egg then travels into the uterus. If you have regular 28-day menstrual cycles, ovulation usually occurs on day 14. However, most women have different menstrual cycle lengths. In general, ovulation occurs 11 to 16 days before your upcoming period.

Ovulation occurs when one of the ovaries releases a mature egg. The egg travels out of the ovary, into the nearest fallopian tube and into your uterus. As the egg moves down the fallopian tube over several days, the lining of the uterus continues to grow thicker and thicker.

It takes about three to four days for the egg to travel toward the uterus. If fertilization is to occur, it must happen within 24 hours of ovulation or the egg degenerates.

It takes about three to four days for the egg to travel toward the uterus. If fertilization is to occur, it must happen within 24 hours of ovulation or the egg degenerates. After ovulation, the luteal phase begins.

The luteal phase

After ovulation, the follicle becomes a hormone-producing structure called the corpus luteum. The cells of the corpus luteum produce estrogen and large amounts of progesterone, with the latter hormone stimulating the uterine lining development in preparation for implantation of a fertilized egg. If you don’t become pregnant, the corpus luteum degenerates about two weeks after ovulation. Because of this, progesterone levels drop and the stimulation for the lining is lost. This causes the lining to shed as a new menstrual cycle starts.

The loss of the corpus luteum can be prevented by fertilization of the egg. If you become pregnant during your period, fertilization will occur within 24 hours of ovulation. About five days after fertilization, the fertilized egg enters your uterus and becomes embedded in the lining. With implantation, cells that will eventually become the placenta begin to produce the “pregnancy hormone” or human chorionic gonadotropin (HCG). It interrupts your menstrual cycle by providing continual stimulation of the corpus luteum to produce progesterone. This prevents the loss of your lining.

During this phase of the menstrual cycle, if you become pregnant, the egg moves into your uterus and attaches to the lining. If you are not pregnant, the lining of the uterus is shed through the vaginal opening. Then, a new menstrual cycle begins.

All about eggs

  • Over the course of a lifetime, you release about 400 eggs in their mature form.
  • The number of eggs that are still contained in the ovaries depends on how old you are.
  • As a 20-week-old female foetus in your mother’s uterus, you have the highest number of eggs you will ever have, approximately seven million eggs.
  • Your body will release the most eggs it ever will before you are even born.
  • At birth, the number of eggs in the ovaries drops to two million. Your ovaries will continue to lose eggs after birth, all the way through puberty.
  • By the time you start puberty, you have between 300,000 and 500,000 eggs in your ovaries.
Always

Sources

  • ACOG Patient Brochure 049.

  • Acog Patient brochure: midlife transition and menopause

  • ACOG Medical Student Education Module 2008

  • Comprehensive Gynecology. Mishell, Stenchever, Droegemueller, and Herbst. 3rd Edition.

  • Textbook of Histology. Bloom and Fawcett 11th edition.

  • Emans Laufer and Goldstein Pediatric and Adolescent Gynecology