The menstrual cycle is the natural cycle of the female body that occurs in the uterus and ovary for the purpose of sexual reproduction. This process is important for the production of eggs and for preparing the uterus for pregnancy, and happens only in fertile female humans and other female primates. The reproductive system repeats a regular menstrual cycle that is controlled by hormones.
A woman’s menstrual cycle is defined as the time between the first day of a period to the first day of her next period. Your menstrual cycle does not necessarily take place once a month. The length of the cycle may vary from woman to woman. A woman’s cycle may last between 21 and 35 days, but the most normal is a 28 day cycle. Most of the time women who have shorter menstrual cycles have a period more often than once a month, and women who have longer periods have fewer periods in a year.
Let’s take a closer look at the basic menstrual cycle phases:
This phase is a woman’s monthly bleeding and is commonly referred to as a period. The first day of the menstrual phase is ‘day one’ of your period. Menstrual blood is shed from the lining of the uterus through the cervix and out through the vaginal opening. The bleeding may last between three and seven days and may be bright red, light pink, or even brownish. The normal amount of menstrual flow during the entire period is about a quarter cup.
The stage following the menstrual phase is the follicular phase. It is during this stage that the hormone oestrogen causes the lining of the uterus to grow, or proliferate. This lining, also called the endometrium, starts to develop to receive a fertilized egg, providing you become pregnant. The increase of another hormone called the follicle-stimulating hormone stimulates the growth of ovarian follicles at this time. Each follicle contains an egg. In the late follicular phase of the cycle, only one of the follicles will remain active.
Around this time the lining of the uterus begins to thicken in response to the increase of oestrogen. The oestrogen level rises dramatically during these days before ovulation and peaks about one day before ovulation. This surge of oestrogen also triggers a spike in the luteinizing hormone and causes the follicle to rupture and release an egg.
The increase in luteinizing hormone triggers this next phase of ovulation. During this stage the mature egg (ovum) is released from the ovarian follicle to the nearest fallopian tube where it will travel slowly down the tube over several days. (Two of these eggs may mature in a month.) The egg then travels into the uterus. At this time the lining of the uterus continues to grow thicker and thicker. It generally takes about three to four days for the egg to travel toward the uterus. Fertilization must take place within 24 hours of ovulation or the egg degenerates.
For women who have regular 28-day menstrual cycles, ovulation occurs on day 14; however, most women have different menstrual cycle lengths so the general occurrence of the ovulation phase is 11 or 16 days before your upcoming period.
After ovulation, the follicle becomes a structure called the corpus luteum which produces oestrogen and large amount of progesterone. The progesterone stimulates the uterine lining development in preparation for implantation of a fertilized egg. If the egg is not fertilized and you do not become pregnant, the corpus luteum degenerates about two weeks after ovulation. Because of this degeneration, the progesterone levels drop and stimulation for the lining is lost. This is what causes the lining to shed and a new menstrual cycle to start.
However, if you do become pregnant during the cycle, fertilization will take place within 24 hours of ovulation. Five days after fertilization, the fertilized egg enters the uterus and is embedded in the lining. With implantation, these cells will eventually become the placenta produce the “pregnancy hormone” or human chorionic gonadotropin (HCG). Fertilization of the egg interrupts the regular menstrual cycle by providing continual stimulation of the corpus luteum to produce progesterone which in turn prevents the loss of your lining.
In short, during the luteal phase if you become pregnant, the egg will move into the uterus and attach to the lining. If you do not become pregnant, the lining of the uterus will be shed through the vaginal opening, causing a new menstrual cycle to begin.