The woman’s menstrual cycle lasts anywhere from 27 to 31 days. Each month during the years between puberty and menopause, a woman’s body goes through a number of physical and hormonal changes in preparation for a possible pregnancy. This is the menstrual cycle.
During the duration of each menstrual cycle, eggs develop from the ovaries and the lining of the uterus thickens. If pregnancy does not occur, then the uterus will shed during what’s known as the period. Once the period is over, the cycle starts again.
A woman’s menstrual cycle is divided into four phases:
- menstrual phase
- follicular phase
- ovulation phase
- luteal phase
The duration of each phase is different and can vary woman to woman.
The Menstrual Phase
This is the first phase of the menstrual cycle and occurs when the mature egg from the last cycle isn’t fertilized. Because pregnancy has not occurred, levels of estrogen and progesterone are low, causing hormonal imbalances and other symptoms such as tender breasts, mood swings, and cramps. The thickened lining of your uterus (which would support the child during pregnancy) is no longer needed, so it sheds and exits through the cervix. During your period, a combination of blood, mucus, and tissue can be released through your vagina. Periods can last anywhere from 3 to 7 days, the duration varying in woman to woman.
The follicular phase overlaps with the duration of your period as it starts on the first day you bleed. The phase ends when you start ovulation. It begins when a signal is sent to your pituitary gland to release follicle-stimulating hormones (FSH). This hormone stimulates your ovaries to produce around 5 to 20 small sacs called follicles, each containing an immature egg.
Normally, only the “fittest” egg will mature, but very rarely, there will be two. The rest of the follicles will be reabsorbed. The maturing follicle will then off a signal for estrogen, eventually causing the thickening of the lining of the uterus. This creates a safe and ideal environment for an embryo to grow. The average follicular phase lasts for about 16 days and can range from 11 to 27 days.
Ovulation is when a woman’s ovary releases an egg that has been matured. The egg will travel down the fallopian tube and towards the uterus. Next, it will wait to be fertilized by sperm. Rising estrogen levels during the follicular phase triggers the pituitary gland, which then releases the luteinizing hormone (LH). This is what triggers the start of ovulation. Contrary to popular belief that a woman can get pregnant anytime, the ovulation phase is actually the only time during a woman’s menstrual cycle in which one can get pregnant.
Ovulation occurs right in the middle of a woman’s menstrual cycle and lasts about a day, the egg will die if it isn’t fertilized.
Once the follicle releases the egg, it will transform into changes into the corpus luteum （a mass of cells that forms in an ovary and is responsible for the production of the hormone progesterone during the phase of early pregnancy). The rise in hormones maintains the ideal thickness of the uterine lining in preparation for a fertilized egg.
If pregnancy does occur, then the production of human chorionic gonadotropin (hCG) will begin. This hormone will help maintain the thickness of the uterine lining and the structure of the corpus luteum.
If pregnancy does not occur, then the corpus luteum will shrink away and be resorbed by the body. This leads to decreased levels of estrogen and progesterone, which triggers the beginning of the period. The uterine lining will begin to shed shortly. Many experience symptoms such as:
- Food cravings
- Weight gain
- Mood changes
The luteal phase lasts for 11 to 17 days. The average length is 14 days.
Various factors can lead to a change in your regular cycle, such as:
- Birth control
- Eating disorders
It is most important to remember that every woman’s cycle is different and that every woman should be making monthly visits to their health care provider.