An Overview of Menstruation

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From your first period (menarche) until your periods stop with menopause, the sole purpose of your monthly cycle is to reproduce. If a fertilized egg does not implant in the wall of your uterus after ovulation, the lining sheds. This is your menstrual period. Menstruation happens every month because you did not get pregnant.

Three different-sized, wrapped menstrual pads on a gray background
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A regular menses occurs, on average, every 28 days or about 14 days after regular ovulation. When the body does not function properly to cause ovulation, menstruation does not come regularly.

Believe it or not, the uterus is more of a bystander in this monthly process. The main players are two structures in the brain—the hypothalamus and the pituitary gland—along with the ovaries. Technically, this is referred to as the hypothalamic-pituitary-gonadal axis. When the interactions of this neuroendocrine trio function properly, ovulation and (if pregnancy doesn’t result from fertilization of the released egg) menstruation happen at regular intervals.

The average ages at which menstruation begins and ends are 12 years and 52 years, respectively.

Menstrual Cycle Stages

A lot of hormonal changes happen in your body leading up to your period. These happen in a predictable way every month and are the driving force behind your menstrual cycle.

Day One

Menstruation is the cyclic shedding of the lining of the uterus. Some might think that menstruation marks the end of their cycle. Even the common term for menstruation—"your period"—makes you think about something coming to a close. In actuality, menstruation marks the first day of a new cycle.

When bleeding starts, the brain has already initiated the hormone changes to trigger a new cycle.

Count the first day of menstruation as cycle day one.

Follicular Phase

As your period begins and the built-up lining from the previous cycle is shed, your brain produces hormones that stimulate the ovaries to release estrogen and prepare an egg for ovulation. This is called the follicular phase.

Under the influence of the rising estrogen levels, the lining of your uterus, or endometrium, begins to thicken or proliferate.


In response to another change in hormone levels from your brain, your ovary releases an egg (oocyte) and ovulation occurs. This usually happens on cycle day 14.

Luteal Phase

The follicle that released the egg now begins to shrink and becomes a structure known as the corpus luteum. It continues to produce estrogen but now begins to produce progesterone as well.

Although both estrogen and progesterone are produced during this part of the cycle, concentrations of the latter dominate. Under the influence of progesterone, the lining of the uterus begins to change in ways to prepare it for pregnancy.

In the last half of the cycle, the uterine lining becomes thicker and more complex with glands, blood vessels, and tissue swelling. These are all changes that prepare it for the process of implantation and pregnancy.

If implantation of a fertilized egg does not occur, the corpus luteum in the ovary continues to shrink away. During this time, estrogen and progesterone levels continue to fall. When this happens, the blood vessels that expanded in the thickened lining constrict and cut off blood flow. The thickened lining, now without blood flow to support it, dies and is shed from the uterus.

Changes in Your Period

Your menstrual cycle is unique to you. How much you bleed, when you start, and for how long is unlike anyone else. (A normal range for menstruation is every 21 to 35 days.) That said, it is also common and very normal for your own menstruation to change from month to month.

Depending on the month, your period may come a few days early/late or your menstrual flow may be heavy/light. Many factors can influence the hormone changes in your body, including:

  • Stress
  • Exercise
  • Weight gain/loss
  • Travel
  • Illness

Some women's cycles are more sensitive to fluctuations in hormone levels than others. Although it is normal for menstruation to vary, it is also normal for it to stay completely regular.

Some women have very heavy bleeding and/or very painful periods. This is not normal. If you are not participating in your normal activities or if you are missing work/school because of painful or heavy periods, discuss it with your healthcare provider.

Missed Periods

It is normal to miss a period occasionally. But once you've started having regular periods, missing your monthly menstruation could be a sign of an underlying medical issue. If you are having sex, however, the most common cause of a missed period is pregnancy.

Consider taking a pregnancy test if you are more than seven days later or have missed a period.

If you are not pregnant, a missed period usually means that ovulation did not occur. The same things that can influence menstrual flow, like stress and exercise, can influence ovulation.

As long as you have determined that you are not pregnant, it is OK to wait another month to see if your period comes. If you miss your period for two or three months in a row, talk to your healthcare provider.


A normal period is not an illness or a disability. There is no medical reason to limit physical activity during your period, including swimming, yoga, and all extreme sports. It is also completely safe, albeit messy, to have sex during menstruation.

There are many options to help you manage your menstrual flow. You may choose a product based on convenience, comfort, or lifestyle. Whatever product you chose, it is important to change it frequently. Consider avoiding products that contain fragrance or perfume, as they can be very irritating.

Some people feel they need to clean out their vagina after their monthly menstruation by using a douche. Not only is this not necessary, but it can also kill off good bacteria in your vagina that keeps it clean and healthy.

A Word From Verywell

Menstruation is a normal function of a woman's body. You should not be ashamed about your period, and your period shouldn't interfere with your lifestyle. If you are having problems with menstruation, see your healthcare provider.

2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Fleming R, Jenkins J. The source and implications of progesterone rise during the follicular phase of assisted reproduction cycles. Reprod Biomed Online. 2010;21(4):446-9. doi:10.1016/j.rbmo.2010.05.018

  2. Petersen JF, Andersen AN, Klein BM, Helmgaard L, Arce JC. Luteal phase progesterone and oestradiol after ovarian stimulation: relation to response and prediction of pregnancy. Reprod Biomed Online. 2018;36(4):427-434. doi:10.1016/j.rbmo.2017.12.019

Additional Reading

By Andrea Chisholm, MD
Andrea Chisolm, MD, is a board-certified OB/GYN who has taught at both Tufts University School of Medicine and Harvard Medical School.