Menstrual Cycle: How Often Periods Occur and Why

The length of time between periods can vary from one person to another

If you are new to getting your period, you may be wondering why girls have periods in the first place, how long a menstrual cycle is, and how often periods happen. You may also be surprised that your first periods didn't start on exactly the same date every month or last for the same amount of time.

The truth is, when you first start menstruating, it's normal for your periods to be a bit irregular. And furthermore, when it comes to how long periods last or how frequently you get them, what's normal for you may not be normal for someone else.

This article covers why girls have periods, how long menstrual cycles are, how often periods happen, and a few reasons for irregular periods.

Tampon and Calendar
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How Often Are Periods?

Every woman is different, but in general, a period is considered "regular" if it starts every 24 to 38 days.

If you are a teenager who has just started menstruating, it's not unusual for your periods to be irregular for the first few years of menstruation. Your period may skip several months, or you may have two that are really close together. Or, you may have spotting every few weeks, with very small amounts of menstrual blood. 

How Long Is a Menstrual Cycle?

If you are an adolescent (between ages 10 to 19) and in your first year of menstruation, your menstrual cycle may be about 32 days long. After your first year of menstruation, your menstrual cycle may shorten to about 28 days long. While there are women who get their periods every 28 days like clockwork, there's a wide range of what's normal.

Why Do Girls Have Periods?

If your periods are irregular, it may help put your mind at ease to first understand how the menstrual cycle works and why girls have periods in the first place.

Every month your body goes through two main phases to complete the menstrual cycle and begin a new one. The length of the menstrual cycle is measured from the first day of bleeding through the last day before your next period starts.

Day 1 of the menstrual cycle is the first day you see any amount of bleeding. This also signifies the first day of the follicular phase, when your hormones trigger an egg to start maturing in preparation for being released. When a mature egg leaves a Fallopian tube, it's called ovulation.

In the luteal phase, the uterine lining is made fully ready to nourish a fertilized egg if it implants—in other words, if you become pregnant. If that doesn't happen, the lining deteriorates and is shed. That's the blood, tissue, and mucus you see when you have your period.

The reason a teen girl's periods may not be following a predictable pattern is simply that the hormone axis between the brain and ovaries that controls ovulation is still developing. After a year or two, when this hormone axis matures, periods should become more regular.

Keeping track of a menstrual cycle calendar or app can help you learn about your pattern and know whether you have "missed" a period or there is something affecting your menstrual cycle. It's also a handy way to make sure you've got pads, tampons, or your menstrual cup on hand before your period starts.

Reasons for Irregular Periods

Once you have a regular cycle and then notice you've missed a period, it could mean you're pregnant (if you've been having sex, of course). If you think that might be the case, you should take an at-home pregnancy test.

When Is My Period Considered Late?

A period is considered late when it is five or more days late compared to your normal menstrual cycle. It is considered a missed period when you do not get your period for more than six weeks.

If your periods begin coming more than 35 days apart, or if you start having them really close together, there are lots of things that could be going on. You could be stressed, exercising too much, have lost a lot of weight, or you may have some sort of hormone imbalance. These are all problems that could cause you to stop ovulating, and, as a result, stop menstruating. Be sure to discuss any such changes over time with your pediatrician or gynecologist.

Frequently Asked Questions

  • Why do females get their period at the same time?

    There are plenty of theories for why period synching may occur. One theory is that menstrual cycles synchronize with the moon's pull on the earth as it orbits our planet. If that's true, women who synchronize with the lunar cycle would also synchronize with each other.

    However, there is no scientific proof to back this theory, and many other studies have debunked period synching altogether.

  • How often are periods during perimenopause?

    An irregular menstrual cycle is typically one of the first signs of perimenopause. Some months you may get a period, some months you may not. Your periods may also be longer or shorter from month to month as well.

  • How do I calculate when my next period will be?

    To calculate when your next period will be, you need to know how long your menstrual cycle is. If your menstrual cycle is 28 days long, then you can count 28 days from the first day of your last period. This will give you an estimate of when your next period will start. You can also use a period tracker app on your smartphone.

  • Why do periods change dates?

    It's normal for the length of your menstrual cycles to change throughout your life. A variety of reasons can affect when your period will start, including age-related hormonal changes, stress, certain medications, excessive exercise, having uncontrolled diabetes, and more.

8 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.
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  2. Adams Hillard PJ. Menstruation in adolescents: what do we know? And what do we do with the information?. J Pediatr Adolesc Gynecol. 2014;27(6):309-19. doi:10.1016/j.jpag.2013.12.001

  3. American College of Obstetricians and Gynecologists. Menstruation in girls and adolescents.

  4. Seattle Children's. Menstrual period - missed or late.

  5. Förster CH, Monecke S, Spiousas I, Hovestadt T, Mitessar O, Wehr TA. Women temporarily synchronize their menstrual cycles with the luminance and gravimetric cycles of the moon. Sci Adv. 2021 Jan;7(5):1358. doi:10.1126/sciadv.abe1358

  6. Jarrar N. Examining the phenomenon of menstrual synchrony at an international co-educational boarding high school in JordanDirasat: Human and Social Sciences. 2020;47(1):1-11.

  7. American College of Obstetricians and Gynecologists. Perimenopausal bleeding and bleeding after menopause.

  8. Eunice Kennedy Shriver National Institute of Child Health and Human Development. What causes menstrual irregularities?.

Additional Reading

By Tracee Cornforth
Tracee Cornforth is a freelance writer who covers menstruation, menstrual disorders, and other women's health issues.