Confession: Up until I was 23 years old, I thought your “menstrual cycle” was your period.

I had no idea my body was on a whole different biological rhythm that repeated itself every ~28 days and resulted in physical and mental shifts week over week. 

But the craziest part is, so many women have found themselves in a similar boat.

Most women don’t get to know their bodies on this level until they’re trying to have a baby. And even then, some women go through the process of having one or multiple children not knowing that they’re operating on an infradian rhythm (i.e., your menstrual cycle).

So what is an infradian rhythm, exactly?

All humans operate off a 24-hour internal biological clock called our circadian rhythm. 

Your circadian rhythm controls your sleep-wake cycle (when you go to bed and wake up), metabolism, energy and alertness, digestion, body temperature, hormone production, and so much more.

But women are unique in that, in addition to our 24-hour circadian rhythm, we also have an infradian rhythm, which is a longer biological rhythm that lasts several days, weeks, or months — the most well-known being the ~28-day menstrual cycle.

Similar to the circadian rhythm, infradian rhythms like the menstrual cycle control various physiological and behavioral processes in the body like hormone production, sexual and social behaviors, metabolic activity, movement patterns, and more. 

The only difference is these patterns are observed week over week or month over month, rather than day by day. 

The four phases of the menstrual cycle 

Everyone’s cycle will differ in its length, regularity, intensity, and related symptoms. Like all things relative to your body, it's individual. 

The goal of your menstrual cycle is to prepare your body for pregnancy in case sperm is available during your ovulatory window. 

It’s also now considered the 5th vital sign for women of reproductive age because of how much it can tell you about what’s going on inside your body based on its consistency, related symptoms and changes, color, intensity, and more. 

Everyone’s “normal” is going to look slightly different, but there are basics that unite and define a healthy cycle, which we’ll outline below. 

1. Menstruation 

What it is: Menstruation, otherwise known as your period, is the first stage of your menstrual cycle and defines the 3-7 days you bleed each month. The day you start bleeding is the first day of a new cycle.

The purpose of your period is to eliminate the unfertilized egg, blood, and tissue that were built up to prepare for a potential pregnancy but are no longer needed, and create space for fresh uterine lining to build in case conception occurs next month.

What happens: During menstruation, your estrogen and progesterone drop, and your body begins to release prostaglandin, a hormone that causes your uterine muscles to contract, helping to shed the inner lining of your uterus (endometrium). 

How you may feel: This is what causes symptoms like cramps, breast tenderness, mood changes, bloating, fatigue, headaches, lower back pain, and more. 

2. Follicular 

What it is: The follicular phase overlaps with menstruation since it too begins on the first day of your period and ends on the day you ovulate. It can range from 10-21 days, but the average length is ~14 days. 

What happens: During your follicular phase, your brain tells your pituitary gland, also known as the “master gland,” to release follicle-stimulating hormone (FSH).

FSH stimulates your ovaries to produce 5-20 small sacs called follicles, each of which contains an immature egg. These immature follicles rapidly begin to grow until a dominant follicle emerges, which contains the strongest and healthiest egg. The rest of the follicles then get reabsorbed back into a woman’s body. 

In some rare cases, such as twins or triplets, there are multiple dominant follicles that make it to the finish line right before ovulation. 

How you may feel: The maturing follicles produce estrogen, which thickens the lining of your uterus in preparation for a potential embryo to grow. These high levels of estrogen are also why many women experience boosted mood, energy levels, and mental acuity during their follicular phase. 

3. Ovulation

What it is: Ovulation occurs around day 14 of your cycle, or roughly a week after your period ends, and lasts 24 hours. 

What happens: Estrogen levels peak, which causes luteinizing hormone (LH) to rise in tandem. LH tells your ovary to release the “winning” follicle from your follicular phase. It then travels down the fallopian tube towards the uterus where it has the potential to be fertilized by sperm. 

Your ovulatory window is the only time in your cycle you can get pregnant, other than one exception — because sperm can live in your uterus for up to 5 days, pregnancy can still result from sex that occurs right before the ovulatory window. 

After a day, however, the follicle will dissolve if it hasn’t been fertilized. 

How you may feel: You’ll know you’re ovulating because you’ll have a slight rise in basal body temperature and experience thicker vaginal discharge that’s clear, stretchy, and higher in quantity. 

Women tend to see an increase in libido, energy, and confidence during ovulation, but some women also experience mild, temporary pelvic discomfort or twinges on one side of their lower abdomen.  

This is often referred to as "mittelschmerz,” which is thought to be caused by the release of the egg from the ovary.

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4. Luteal   

What it is: The luteal phase is the final phase of your menstrual cycle and typically lasts for 11-17 days, with the average length being 14 days.

What happens: Right after ovulation, estrogen and LH levels drop and the follicle that contains your egg transforms into a corpus luteum, a temporary group of cells that forms in your ovary to produce progesterone (and some estrogen) to keep your uterine lining thick and prepare your body for a potential pregnancy. 

If you do get pregnant: Your body will start producing human chorionic gonadotropin (hCG) to sustain the corpus luteum so your uterine lining stays thick. This is the hormone that’ll show up on a positive pregnancy test.  

If you don’t get pregnant: The corpus luteum will shrink away and get reabsorbed back into your system. 

This triggers a large decline in your estrogen and progesterone levels, which creates the onset of your period to get rid of the uterine lining and unfertilized egg that wasn’t necessary this time around. 

How you may feel: Because of the sudden drop in estrogen and progesterone, you may also experience PMS (premenstrual syndrome) symptoms such as breast pain and tenderness, mood and libido changes, headaches, trouble sleeping, food cravings, water retention, fatigue, etc. 

What can affect your menstrual cycle 

Your cycle can be affected by:

  •  Birth control
  • Eating disorders and, or dieting

  • Extreme exercise

  • Stress

  • Weight fluctuations (gain or loss)

  • Uterine fibroids

  • PCOS

  • Thyroid dysfunction (hyper, hypo, or Hashimoto’s)

  • Medications

  • Time zone changes

  • Hormonal imbalances 

  • Other medical conditions like autoimmune diseases

  • Pregnancy, perimenopause, and menopause (kind of a no-duh, we know)

Signs something’s off with your menstrual cycle 

Because the menstrual cycle is a vital sign for women of reproductive age, if you notice any of these changes in yours, we would recommend reaching out to your physician. 

  • You’ve missed one or more periods (and you’re not pregnant)

  • Your periods have stopped entirely 

  • You bleed for more than 7 days 

  • Your periods don’t come on time 

  • Your periods differ in their lengths 

  • Your periods are less than 21 days or more than 35 days apart

  • You bleed between periods (heavier than spotting)

  • You bleed through a tampon or pad in less than 1-2 hours

  • You pass clots the size of a quarter or larger

  • You have debilitating menstrual cramps and, or pelvic pain that forces you to miss work, school, physical education classes, events, etc.

  • You’ve seen changes in the color, consistency, or odor of your menstrual blood

As women, we have been trained through society to view our menstrual cycles, especially our period, as a burden. But when you understand what’s going on inside your body, you can begin to work with it and leverage the strengths that come with each phase to your advantage.

Caroline McMorrow is a Content Manager at Rescripted.