The average menstrual cycle is about 28 days. This means that about 28 days pass between the first day of your period and the first day of your next period.

Not everyone has this textbook cycle, though. You may find that your periods typically occur every 21 to 35 days.

Periods that are closer together or further apart aren’t always cause for concern.

Tracking your menstrual patterns may help you better understand your overall cycle as well as reveal symptoms you should discuss with a doctor or other healthcare provider.

Menstrual flow lengths vary and may last anywhere between two and seven days. Flow is generally heavier in the first days and may trail off to light or spotting in the final days.

There are many situations where your period may come more frequently than every 21 days.

People in perimenopause, for example, may experience shorter, more irregular cycles until they reach menopause.

Other factors that may shorten cycle length include:

  • stress
  • temporary illness, such as the flu
  • significant weight changes
  • hormonal birth control
  • uterine fibroids
  • lack of ovulation (anovulation)

Oftentimes, your cycle will resolve on its own.

If you’re still experiencing shorter cycles (having more than one period in a single month), see a doctor after six weeks of irregularity.

They can determine what’s causing your irregularity and advise you on any next steps.

Menstruating individuals usually begin having a period between ages 9 and 15. The average person experiences at least four periods during their first year of menstruation.

This number will gradually increase with time, with the average adult having at least nine periods a year. This means that some periods may naturally occur more than 35 days apart.

Occasional lateness may also be caused by:

  • stress
  • intense exercise
  • significant weight changes
  • hormonal birth control
  • perimenopause

Chronic lateness may be caused by an underlying condition. Polycystic ovary syndrome (PCOS), for example, can cause:

  • irregular periods
  • excess hair growth on the body
  • unexpected weight gain

Premature ovarian failure may also cause irregular or occasional periods in menstruating individuals under age 40.

Pregnancy is another possibility. If you’re sexually active, it may be a good idea to take a home pregnancy test.

If you suspect pregnancy or another underlying condition is to blame, make an appointment with a doctor. They can assess your symptoms and advise you on any next steps.

Menstruation

The first day of your flow is day one of your cycle.

During this phase, your uterine lining is shed through your vagina over a period of three to seven days. Your menstrual flow contains blood, uterine tissue, and cervical mucus.

Follicular phase

The follicular phase begins with menstruation and ends before an egg is released from your ovaries.

During this time, your brain sends signals to your body to produce follicle-stimulating hormone. Your ovaries produce between 5 to 20 follicles that contain immature eggs.

Ovulation

Ovulation usually occurs between days 10 and 14 of your cycle.

The increase in estrogen prompts your body to produce luteinizing hormone. It triggers the release of a mature egg for potential fertilization.

This egg is released into your fallopian tube. It’ll stay there for about 24 hours. If the egg isn’t fertilized, it’ll be shed in your menstrual flow.

Luteal phase

The luteal phase starts after ovulation and ends with the first day of your period. It lasts approximately 12 to 15 days.

During this time, your body produces progesterone. This causes your uterine lining to thicken in preparation for implantation and pregnancy.

If pregnancy doesn’t occur, your progesterone levels will drop. This causes your uterine lining to shed, signaling day one of your new menstrual cycle.

Tracking your period can be as simple as writing down when your flow starts and ends on a calendar.

If you’re experiencing irregularity, you may also find it helpful to record:

  • Flow volume. Think about how often you change your pad, tampon, or other protection. The more you change it, the heavier your flow. Also take note of any color or texture changes.
  • Aches and pains. Cramping — especially outside of menstruation — may be a sign of another underlying condition. Make sure you record the timing, point of origin, and severity.
  • Unexpected bleeding. Also note any bleeding that occurs outside of your expected window of menstruation. Make sure you record the timing, volume, and color.
  • Mood changes. Although it can be easy to write mood changes off as PMS, they may point to another underlying condition, especially when coupled with menstrual irregularity.

There are also free apps that allow you to record this information on the go. Consider checking out:

The more you log, the more these apps can tell you about predicted menstruation dates, your fertile window, and more.

Although occasional changes are often tied to stress and other lifestyle factors, consistent irregularity may be a sign of an underlying health condition.

See a doctor or other healthcare provider if:

  • You haven’t had a period for three months.
  • You regularly have a period more than once every 21 days.
  • You regularly have a period less than once every 35 days.
  • Your periods last for more than one week at a time.
  • You soak through one or more menstrual products per hour.
  • You pass blood clots the size of a quarter or larger

Tracking your menstrual flow and other symptoms that occur throughout your cycle can help your provider determine the underlying cause.

This may take a bit of trial and error, so be open with your provider and give it time.