The menstrual cycle is divided into 4 stages:
On average, girls start menstruating at about 12 years old and continue until they reach age 40. Although the average menstrual cycle is 28, this varies from woman to woman. Some women have shorter cycles over 21 days and others have longer cycles over 45 days.
Learn more about the menstrual cycle and what to expect during each stage.
Stage 1: menstruation
Menstruation occurs once a month as the thickened uterine lining sheds and is eliminated through the vagina along with blood, mucus, and cells of the uterus. Levels of the hormones estrogen and progesterone are at their lowest during this phase.
The length of a menstrual phase lasts between 3-7 days. On average, 30-40 mL of blood is lost, with some women losing up to 60 mL. Some women have heavy periods, whereas some have very minimal discharge to spotting.
Most women experience some symptoms with every period such as:
- Abdominal cramps
- Lower back pain
- Nausea and vomiting
- Tender breasts
- Mood swings
Women use pads, tampons, and menstrual cups to collect the discharged blood. The use of tampons is associated with the risk of toxic shock syndrome, so it is highly recommended to change every 4-8 hours.
Stage 2: follicular phase
The follicular phase, also known as the proliferative phase, begins with the first day of menstruation and ends with ovulation. The pituitary gland secretes follicle-stimulating hormone (FSH) in response to a signal from the hypothalamus. FSH causes the ovary to develop 5-20 follicles (tiny nodules or cysts) that bead on the surface.
Each follicle contains a developing egg. Only one follicle will typically mature into an egg, whereas the others will be absorbed by the body. This usually occurs on day 10 of a 28-day cycle. The mature follicles increase estrogen levels, which causes the uterine lining to thicken. This thickened uterine lining aids in the implantation of the embryo and formation of vital nutrients for normal growth.
Follicles can live for an average of 16 days, although this can range from 11-27 days depending on the cycle.
Stage 3: ovulation
Ovulation is the discharge of a mature egg from the ovary. This normally occurs in the middle of the cycle, about 2 weeks before menstruation begins.
The growing follicle increases the level of estrogen in the body during this period. When the hypothalamus in the brain detects these increased levels, it releases gonadotropin-releasing hormones, which stimulates the pituitary gland to produce more luteinizing hormones (LH) and FSH.
The elevated amount of LH, known as LH surge, causes ovulation to occur within 2 days. Waves of tiny, hair-like projections guide the egg into the fallopian tube and toward the uterus. The average egg has a lifespan of about 24 hours. It will die unless it comes into contact with a sperm during this time. If an egg comes in contact with sperm, fertilization occurs, leading to pregnancy.
If you want to get pregnant, it is crucial to understand how ovulation works as well as the “fertile window” in the menstrual cycle, which typically occurs 10-16 days before your next period. If you have a shorter or longer menstrual cycle, you should talk to your doctor about your fertile window to increase your chances of getting pregnant.
Ovulation or release of the egg is usually painless, but some women may experience lower abdominal pain toward the side where the egg is released. This is a rare condition known as Mittelschmerz syndrome. Some women may experience other symptoms during ovulation, such as:
- White discharge
- Raise in basal body temperature
Stage 4: luteal phase
The egg breaks from its follicle during ovulation, yet the ruptured follicle remains on the ovary's surface. The follicle then develops into a structure called the corpus luteum over the next 2 weeks. This structure begins to release progesterone and trace levels of estrogen. This hormone cocktail keeps the uterine lining thicker while waiting for a fertilized egg to implant.
When a fertilized egg implants in the uterine lining, it produces the hormones required to sustain the corpus luteum. This includes human chorionic gonadotropin, the hormone found in a pregnancy urine test. The corpus luteum continues to produce increased quantities of progesterone required to maintain the thicker lining of the uterus.
If there is no pregnancy, the corpus luteum withers and gets reabsorbed, usually around day 22 of a 28-day cycle. The uterine lining falls away due to a reduction in progesterone levels. This is referred to as menstruation. The cycle then begins again.
Although the length of a menstrual cycle varies from 24-35 days or so, the luteal phase will always be constant at 14 days. It is the follicular phase that varies in duration.
What is PMS?
Premenstrual syndrome (PMS) is a condition that follows the luteal phase in the absence of pregnancy. This phase begins 7-10 days before menstruation and usually ends a few hours later. About 50% of women experience PMS, with 5% of women experiencing a severe form of PMS called premenstrual dysphoric disorder.
Symptoms of PMS may include:
What hormones are involved in the menstrual cycle?
- Follicle-stimulating hormone (FSH):
- Promotes egg formation and estrogen release
- Luteinizing hormone (LH):
- Stimulates egg release (ovulation)
- Stimulates the synthesis of the hormones estrogen and progesterone
- Stimulates the growth of the uterine lining
- Inhibits FSH
- Aids in ovulation by stimulating LH
- Inhibits LH after ovulation
- Keeps the uterine lining in place
- Inhibits LH after ovulation
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The Normal Menstrual Cycle and the Control of Ovulation: https://www.ncbi.nlm.nih.gov/books/NBK279054/
Normal Menstruation: https://my.clevelandclinic.org/health/articles/10132-normal-menstruation
What is ovulation? https://www.hopkinsmedicine.org/health/wellness-and-prevention/menstrual-cycle-an-overview