Your Menstrual Cycle Explained: The Periods Facts You Never Knew You Never Knew

Woman looking at her menstrual cycle in the Fitbit app

Maybe the first (and last) time you learned about the menstrual cycle was in sixth grade health class. Or maybe you’re confident you know the basics but still feel clueless about what’s “normal” and what’s not. Whatever your status, the fact remains: The menstrual cycle is complex. Despite the fact that half the population is designed to have a period, many people still don’t understand why women bleed or all the steps that lead up to that monthly (or so) event.

“The menstrual cycle is as complicated—and as beautiful—as listening to an orchestra play a symphony,” says Kate White, MD, MPH, assistant professor of obstetrics and gynecology at Boston University. “All of the instruments need to be in tune, and the musicians working together, to make such a wondrous sound. So it’s no wonder that my patients are a bit mystified by the process—it seems like magic!”

Whether you have “regular” periods, problematic periods, or no periods at all, knowing more about menstrual health can help you and the women in your life feel better educated and more empowered to make informed decisions.

Below, everything you’ve ever wanted to know about the menstrual cycle.

What is the Menstrual Cycle?

The menstrual cycle refers to the monthly changes a woman’s body goes through to prepare for the possibility of pregnancy. Although the first day of your period is considered day one, the cycle really kicks off when certain hormones shift, the lining of the uterus thickens, and one of your two ovaries releases an egg—a process known as ovulation.

After ovulation, the egg travels down one of the two fallopian tubes that lead to the uterus. If it gets fertilized by a sperm, then the egg implants in the thickened lining of the uterus and a pregnancy begins.

If the egg isn’t fertilized, then hormone levels drop. This signals to the uterus that it’s time to shed its lining through the vagina. That shedding of blood and tissue is called menstruation, or a period. Menstruation usually starts between the ages of 10 and 15 and permanently stops at menopause (the average age of which in the United States is 52).

What Hormones Are Involved in the Menstrual Cycle?

Believe it or not, your period starts in your brain! A part of your brain known as the hypothalamus produces important hormones. One of these hormones—gonadotropin releasing hormone (GnRH)—stimulates your pituitary gland to produce follicle stimulating hormone (FSH). Just like its name suggests, FSH stimulates the growth of fluid-filled sacs in the ovaries called follicles. Each follicle contains a single egg.

As the follicles mature, the ovaries produce more estrogen, the main female sex hormone. The increased estrogen causes the pituitary gland to produce more luteinizing hormone (LH), which helps the eggs mature and eventually triggers one of them (well, in most cases it’s just one) to be released during ovulation.

“Most people think of estrogen as the female testosterone counterpart,” says White. “But there are two hormones that dominate our cycle: estrogen and progesterone.” Progesterone, which your body needs to support a healthy pregnancy, comes into play after ovulation when the cells that are left behind in the follicle form a temporary structure called the corpus luteum.

LH stimulates the corpus luteum to make more estrogen and progesterone. If the egg that’s released during ovulation isn’t fertilized and you don’t become pregnant, the corpus luteum breaks down, progesterone drops, and menstruation begins.

What Happens During a Typical Menstrual Cycle?

Every woman’s body is different, but the average cycle lasts 21 to 35 days and consists of two phases: the follicular phase and the luteal phase.

The Follicular Phase

The follicular phase encompasses the first half of your cycle. This kicks off on day one—the first day of your period—and ends at ovulation. During this time, you bleed (usually no more than eight days) and FSH stimulates a small number of follicles to form on your ovaries. Each one of these follicles contains a single egg. One of these follicles matures while the lining of the uterus starts to thicken.

In a 28-day cycle, ovulation typically occurs around day 14, but every woman is different and the exact timing can vary from cycle to cycle. This is when LH and FSH increase, which stimulates the mature follicle to release its egg. The egg then travels toward the uterus through one of the fallopian tubes.

The Luteal Phase

During this second half of your cycle, the levels of LH and FSH decline, and progesterone rises. If a sperm fertilizes the egg, then that egg/sperm combo will implant in the uterus and begin developing into a baby. If the egg isn’t fertilized, estrogen and progesterone begin to drop around day 21 or 22, and the next cycle begins. The unfertilized egg will break apart and be shed along with the uterine lining.

While the follicular phase and luteal phase are each usually about two weeks long, the length of the follicular phase can vary from cycle to cycle. Certain factors like stress, dietary changes, and even jet lag can sometimes delay ovulation. This delay may cause progesterone to peak later in the month, which can then cause menstruation to occur a few days late. Don’t worry: It’s perfectly normal for your cycle to vary in length by as many as seven to nine days from one month to the next.

How Long Does the Average Period Last?

Menstrual bleeding can last anywhere from two to eight days and can occur every 21 to 35 days. Most women lose an average of about 30 milliliters (about two tablespoons) of blood during each period, but that amount can vary quite a bit. “I know it feels like much more than that,” says White. “It’s the spreading out of the bleeding that makes it feel so messy and like so much.”

Different types of feminine products may absorb different amounts of blood, but if you routinely need to change your pad or tampon after less than two hours or you pass clots larger than the size of a quarter, talk to your doctor.

In some cases, women experience cycles that are shorter or longer than average. If your cycle lasts longer than 35 days or is shorter than 21 days, consult your doctor.

What is the Benefit of Tracking My Period?

Because hormones fluctuate all month long, tracking your cycle can help you recognize any recurring symptoms or irregularities and identify menstrual patterns linked to everyday activities, like sleep and exercise. Knowing how your performance in these activities varies throughout your cycle can also help you modify your lifestyle habits so you can try to be at your best and perform at your peak.

“Having this level of individualized information means [the technology] can act like a coach for you,” says White, “guiding you to when you need to sleep more or exercise differently.”

What Are Some Common Menstrual Irregularities?

Every woman is different, but there are a few characteristics that define a typical menstrual cycle: It lasts 21 to 35 days, menstrual bleeding lasts two to eight days, and bleeding is limited to less than 60 milliliters per cycle (meaning you don’t have to change a pad or tampon more than every two hours).

Some women may experience irregularities outside of these characteristics, including:

  • Premenstrual syndrome (PMS): Many women feel some minor discomfort like bloating and breast soreness just before their period. PMS, on the other hand, can cause intense physical or emotional symptoms like acne, headaches, anxiety, depression, and more.
  • Premenstrual Dysphoric Disorder (PMDD): This is a severe form of PMS characterized by a markedly depressed mood, irritability, and other symptoms that occur during the last week of the luteal phase.
  • Amenorrhea: The absence of a period.
  • Abnormal uterine bleeding: Bleeding for more than a week and/or soaking through one or more pads or tampons every hour for several hours in a row, spotting between periods or after sex, etc.
  • Dysmenorrhea: Painful periods caused by intense uterine contractions that result in cramps that can be felt in the lower abdomen, back, or thighs.

What Can Cause an Irregular Period?

There are a lot of reasons why your period may become irregular. Some common causes include:

  • Hormonal birth control
  • Pregnancy
  • Breastfeeding
  • Eating disorders and/or extreme weight loss
  • Excessive physical activity
  • Endometriosis
  • Polycystic ovary syndrome (PCOS)
  • Premature ovarian failure
  • Pelvic inflammatory disease (PID)
  • Uterine polyps or fibroids
  • Perimenopause

What Should I Do If My Period is Irregular?

In general, if you ever have questions about your cycle, you should talk to your doctor. If your period routinely comes more often than every 21 days or less often than every 35 days, and/or you’re having unusually heavy bleeding, make an appointment. You should also consult with your doctor if your period goes missing for several months in a row or if you experience severe cramps in your lower abdomen, back, and/or thighs just before or during your period.

6 Comments   Join the Conversation

6 CommentsLeave a comment

  • Which of the fitbit devices are able to track menstrual cycles? I am unable to see an area for it in the app either, so any help on this to help me to choose the better FitBit for all my tracking would be appreciated, thanks

    • First and foremost, you should make sure you have the latest Fitbit app. Once you upgrade, the Female Health Tracker will appear as Dashboard tile.
      See this article: https://blog.fitbit.com/female-health-tracking

      If you don’t have a Fitbit watch, e.g. I only have the Charge 2, the Female Health Tracker might not appear on your device, but you should have access to tracking your menstrual cycles through the app.

      Hope this helps =)

  • I’d like to see the ability to add more PMS options into the log. I need to be able to track my cravings. My weight fluctuates by quite a bit and it seems to run on a monthly cycle. So it would be good to see the correlation.

  • Hi Michelle. I have been experiencing bleeding mid month, every months for the last 12 months even though im on the same pill iv been on for 15 yrs. Drs can’t seem to find the answer and just tell me im stressed. I sought help from a gyno and she told me im just getting old. Can u shed any light?

  • I think the average blood loss of 30mL is incorrect. Most Menstral Cups will hold 20-25mL on their own and need changing more than once. I know I have a heavy flow period and I fill my cup 3-5 times a day over the heaviest two days of my cycle! You can do the maths on that one. I Googled “average blood loss for period” and the first result says “Most women will lose less than 16 teaspoons of blood (80mL) during their period, with the average being around 6-8 teaspoons. Heavy menstral bleeding is defined as losing 80mL or more in each period, having periods that last longer than 7 days, or both.”

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