What is menstruation (also known as a menstrual period or bleeding)?

When you menstruate, your body discards the monthly buildup of the lining of your uterus (womb). Menstrual blood and tissue flow from your uterus through the small opening in your cervix and pass out of your body through your vagina. To menstruate regularly, a woman first ovulates, and if she does not get pregnant, then menstruation starts about 14 days after ovulation.

During the monthly menstrual cycle, the uterus lining first sheds and then builds up again to prepare for ovulation and implantation (pregnancy). If you do not get pregnant, estrogen and progesterone hormone levels begin falling and when progesterone/estrogen levels become very low which signals your body to begin menstruating.

A normal menstrual period lasts between 2-6 days. Shorter periods or periods that are longer or heavier should be checked to make sure there is no issue.

• Your menstrual periods and cycles are your "fifth vital sign".
• Learn hear the names of different irregular cycles

Check out our week-by-week pregnancy calendar and interactive pregnancy wheel to follow your pregnancy day-by-day.

What is a menstrual cycle?

A full menstrual cycle is the time period between the first day of your menstrual period, and the next time you begin your menstrual period.

Cycle day 1 of the menstrual bleeding is referred to as Day 1 of the menstrual cycle. The length of the cycle is measured from Day 1 of one cycle to Day 1 of the next cycle.

In over 70% of women, the menstrual cycle length is between 26-32 days, with older women having somewhat shorter cycles and younger women having a little longer cycles.

Your menstrual cycle length and the day of ovulation are directly related. The next menstrual period begins at the end of the luteal phase, which is on average about 14 days after you ovulate, though in about 2/3 of women the normal luteal phase can last between 13 to 16 days. If ovulation occurs on approximately day 14 of a menstrual cycle and the luteal phase is 14 days, then the next menstrual period starts about 14 days later and the cycle length is 28 days.

The changes associated with ovulation and the menstrual cycle are brought on by fluctuations in hormones at different times of the month. Most menstrual cycles are between 26-32 days long. A variation of a few days more or less can be quite normal as well as small variations from cycle to cycle. 

Ovulation occurs when a mature egg is released from the ovary. It is then picked up by the fallopian tube and is available to be fertilized. The lining of the uterus has thickened to prepare for a fertilized egg. If no conception occurs, the uterine lining, as well as blood, will be shed. The shedding of an unfertilized egg and the uterine wall is the menstrual period. The menstrual cycle can be divided into the following two parts: the ovarian cycle and the uterine cycle.

What is the ovarian cycle?

The ovarian cycle involves changes in the ovaries, and can be further divided into two phases: 

Follicular Phase (Phase 1): The follicular phase (days 1 through 13) is the time from the first day of your period until ovulation when a mature egg is released from the ovary. It's called the follicular phase because the growth or maturation of the egg is taking place inside the follicle, a small sac where the egg matures. Ovulation occurs around day 14 of the cycle, in response to a surge of luteinizing hormone (LH) when the egg is released from the ovary. This first half of the cycle can differ greatly for each person lasting anywhere from 7 days until 40 days.

Luteal Phase (Phase 2): The luteal phase (days 14 through 28) is the time from when the egg is released (ovulation) until the first day of your next period. The luteal phase has a more precise timeline and usually is only 12-16 days from the day of ovulation. This ultimately means that the day of ovulation will determine how long your cycle is.

Your menstrual periods and cycles are a window into your health

You may think the only thing your period can tell you is whether or not you're pregnant—or if need to take pain medication because your menstrual cramps hurt. But your menstrual cycle and your periods can actually clue you in to many other health issues, too. 

According to new research from the American Heart Association, women whose periods start at age 10 or younger—or 17 and older—have a higher risk of developing heart disease, stroke, and health complications related to high blood pressure. Women who experience their first menstrual cycle at 13, however, have the least risk of experiencing those conditions. Granted, that doesn't mean that your period causes any of these problems—or that you're doomed to experience them if you're an early or late bloomer. But it's still a good reminder that your flow can serve as an indication of other things going on in your body.

• Anovulation - No ovulation
• Asherman syndrome - Not enough or no bleeding
• Uterine Polyps - Usually too heavy bleeding
• Fibroids or Myomas - Benign tumors of the uterus
• Ectopic Pregnancy
• Polycystic Ovarian Syndrome (PCOS)

  Women who have PCOS suffer from a hormone imbalance that leads to problems with their periods and their ability to get pregnant (it can also lead to excess facial hair, obesity, ovarian cysts, infertility troubles, and other side effects). Luckily, hormonal birth control can help treat the syndrome.
• Anemia
  If you have a super heavy flow, you may be anemic. Anemia is a condition in which your blood lacks red blood cells or hemoglobin (the main stuff red blood cells are made of). When this happens, your body doesn't absorb enough oxygen, making you sluggish and giving you other unpleasant symptoms, such as headaches and dizziness. Since this can also cause an iron deficiency, your doc may recommend supplements.
• Uterine Cancer

  Some of the earliest noticeable signs of uterine cancer are "irregular bleeding, bleeding after menopause, and bleeding in between menses," says Dweck. Experiencing pain during sex can also be a tip-off. Of course, these can be signs of other, less serious conditions, too—but it's worth checking with a doctor if you notice these symptoms.
• Thyroid Disease

  Since your period is so driven by hormones and your thyroid plays such a large role in hormone production and regulation, noticeable changes to your period—like a lighter or heavier flow—could be an indication of thyroid issues, says Dweck. Thyroid tests can tell you if that may be behind your irregular flow.
• Pituitary Tumor
  If you miss a period, it could obviously be pregnancy. But if you also notice milky breast discharge, it might mean that you have a pituitary tumor, a benign or cancerous mass on your pituitary gland. Headaches are also a possible symptom, says Dweck. These are most often found in older adults (so don't go into instant freakout mode and self-diagnose), but they can occur at any age.
• An Eating Disorder

  When you're not eating right, it can affect your flow. "Absent menses in the presence of rapid or notable weight loss, with a low BMI, and/or excessive exercise," point doctors to eating disorders such as anorexia or bulimia, says Dweck. If this is behind your period changes, you likely already suspect that you may be suffering from disordered eating. But if you haven't come to terms with it yet, hopefully this will be the nudge you need to seek help.

What are the cervical mucus changes during a menstrual cycle?

Cervical mucus changes according to ovulation and your menstrual cycle.

• Cycle day 1-5: Menstrual bleeding
• Menses cycle 6-9 (5-8 days before ovulation): Dry; little or no mucus
• Cycle day 10-12 (2-4 days before ovulation): Sticky thick mucus, becoming less thick and whiter
• Cycle day 13-15 (1-2 days before and after ovulation):  Egg-white or "spinnbarkeit" mucus (most fertile time): thin, elastic, slippery, stretchy, clear
• Cycle day 16-21 (2+ days after ovulation): Sticky, thick mucus (Less fertile/infertile)
• Cycle day 22-28: Dry mucus

How does stress affect your period?

The old thought that stress can affect your period is partly true. Stress can affect your ovulation which ultimately determines when your menstrual period will come, but stress around the time of an expected period will not make it late (because it was already determined when it would come 12-16 days earlier!).

Find out why estimating the correct pregnancy due date is important.

What is the uterine cycle?

The uterine cycle involves changes in the uterus. It occurs in tandem with the ovarian cycle, and is divided into two phases:

The proliferative phase (days 5 through 14): This is the time after menstruation and before the next ovulation, when the lining of the uterus increases rapidly in thickness and the uterine glands multiply and grow.

The secretory phase: (days 14 through 28): This is the time after ovulation. When an egg is not fertilized, the corpus luteum gradually disappears, estrogen and progesterone (hormone) levels drop, and the thickened uterine lining is shed. This is your period.

When are you most fertile?

A monthly cycle is measured from the first day of the menstrual period until the first day of the next period. On average, a menstrual cycle normally lasts between 28-32 days, but some people may have much shorter cycles or much longer ones. The cycle length depends all on when you ovulate. Ovulation can occur at various times during a cycle and may occur on a different day each month.

Ovulation can be calculated by starting with the day the last menstrual period (LMP) starts or by calculating 12-16 days from the next expected period. Most women ovulate anywhere between Day 11 — Day 21 of their cycle, counting from the first day of the LMP.

The fertile window: When can I get pregnant?

The fertile window lasts about 5-6 days long and it includes the day of ovulation and the 4-5 days before that day. Calculate your fertile window here  

Why is tracking ovulation useful?

Tracking ovulation and the menstrual cycle can help you get a better idea of when pregnancy can and cannot occur during a monthly cycle. One way to track when ovulation occurs is to study the changes in cervical mucus and use a basal thermometer. Cervical fluid will change to a wet, slippery substance that resembles "egg whites" just before ovulation occurs and until ovulation is over. A thermometer helps track a body temperature rise, which signals that ovulation has just occurred. Another way to track ovulation is through ovulation kits and fertility monitors. Once ovulation has occurred, there is nothing you can do to increase your chances of pregnancy. Your next step is to begin watching for early pregnancy symptoms.

Did you know?

1. An egg (ovum) lives for no longer than 12-24 hours after ovulation, once it leaves the ovary.
2. Normally only one egg is released each month during ovulation, but occasionally two or more eggs are released.
3. Ovulation (and then your menstrual cycle length) can be affected by stress, illness, or disruption of normal routines.
4. Some women may experience some light spotting during ovulation.
5. Implantation of a fertilized egg normally takes place 6-12 days after ovulation.
6. Each person with ovaries is born with millions of immature eggs that are waiting for ovulation to begin.
7. A menstrual period can occur even if ovulation has not occurred.
8. Ovulation can occur even if a menstrual period has not occurred.
9. Some women can feel a bit of pain or an ache near the ovaries during ovulation. This is called "mittelschmerz."
10. If an egg is not fertilized, it disintegrates and absorbs into the uterine lining.
11. Sperm can live inside the vagina and uterus for up to 5 days after intercourse.