The number of days between periods can vary more than many people realize. A cycle that was previously 28 days might occasionally stretch to 31 days, shorten to 26 days, or fluctuate for several months in a row. These changes can happen because of stress, hormone fluctuations, medication, lifestyle changes, sleep disruptions, or underlying medical conditions.
In most cases, small variations are completely normal and not a sign of a serious problem. Your menstrual cycle is considered a vital sign of health because it provides insight into hormonal balance, reproductive functioning, and overall well-being. Understanding your menstrual pattern can help you detect changes early, support fertility planning, and know when to seek medical attention.1

A regular menstrual cycle begins at puberty and continues until menopause, which usually occurs between ages 45 and 55.2 The cycle repeats every month and prepares the body for a possible pregnancy. If no pregnancy occurs, the uterine lining sheds, resulting in menstrual bleeding. The hormonal rhythm behind this process is powerful yet sensitive, meaning it can be affected by even small changes in lifestyle, health, or the environment.

What Is the Average Menstrual Cycle?

Most adults experience cycles that range between 21 and 34 days.3 Adolescents often have cycles between 21 and 45 days because their hormones are still stabilizing. Menstrual bleeding generally lasts two to seven days. On average, most people lose around two to three tablespoons (tbsp) of blood during each menstrual period, although some individuals bleed more heavily while others bleed less.4
Ovulation usually takes place between days 11 and 14 of a standard 28-day cycle.5 This is when a mature egg is released from one of the ovaries and becomes available for fertilization. If fertilization does not occur, hormone levels drop, and menstruation begins, marking the start of a new cycle.

The average age of menarche—the first menstrual period—is between 12 and 13 years old in the United States. People then continue to menstruate for nearly 40 years before reaching menopause, which is defined as 12 consecutive months without a period.4

Because the menstrual cycle is influenced by hormones, stress, sleep, diet, illness, and physical activity, it is perfectly normal for cycle length to shift occasionally without indicating a medical concern.

How To Track Days Between Periods

Tracking your cycle helps you understand what is normal for your body and whether any changes should be discussed with a healthcare provider.
You can use a calendar, journal, or a period-tracking app to record the first day of menstruation each month. After several months, patterns become clearer. Tracking also helps identify signs of ovulation, such as changes in cervical mucus, mild pelvic discomfort, or temperature shifts.

what to record when tracking cycles

  • the first day of each period

  • how long bleeding lasts

  • the number of pads or tampons used

  • how often you must change menstrual products

  • when your flow is heaviest or lightest

  • what PMS symptoms occur and when

  • whether cramps interfere with daily activities

This information can help your healthcare provider diagnose any underlying hormonal, thyroid, or reproductive concerns. Tracking also helps you estimate ovulation, which is useful whether you are trying to conceive or trying to avoid pregnancy.4

What Are Irregular Menstrual Cycles?

Even though a 28-day cycle is often described as “standard,” periods rarely follow the exact same schedule each month. If your cycle length consistently falls between 21 and 35 days, you are considered regular.
Occasional longer or shorter cycles—such as one month being 31 days and the next being 27 days—are still normal.

However, about 25% of people experience irregular menstrual cycles, meaning their periods come unpredictably or outside the 21–35 day range.1 Irregularities may include:

amenorrhea
No period for three months or more (not including pregnancy). Can indicate hormonal imbalance, extreme exercise, low weight, or certain medical conditions.

oligomenorrhea
Cycles longer than 35 days.

polymenorrhea
Cycles shorter than 21 days.

prolonged or short menstrual bleeding
Periods lasting more than eight days or fewer than two days.

intermenstrual bleeding
Spotting or bleeding between periods.

If your cycle becomes irregular suddenly or gradually becomes more unpredictable, it is worth tracking carefully and discussing with your clinician.

Risk Factors for Irregular Menstrual Cycles

Many factors increase the likelihood of irregular cycles, including hormonal shifts, lifestyle habits, medications, and underlying health conditions.1

common risk factors

  • age: teenage years and perimenopause naturally cause irregular cycles

  • medications: antidepressants, antipsychotics, and epilepsy medications may disrupt hormones

  • eating disorders: anorexia and severe calorie restriction can suppress menstruation

  • high prolactin levels: excess prolactin affects ovulation and cycle regularity

  • hormonal birth control: pills, patches, injections, IUDs, and implants may lighten, delay, or stop periods entirely

  • perimenopause: hormonal fluctuations in your 40s and 50s make cycles less predictable4

medical conditions that may cause irregular periods

  • Asherman’s syndrome (uterine scarring)

  • congenital adrenal hyperplasia

  • Cushing’s syndrome (increased cortisol levels)

  • untreated diabetes

  • primary ovarian insufficiency (POI)7

  • thyroid disorders (hypothyroidism or hyperthyroidism)8

If you suspect one of these issues, tracking your cycles and symptoms is an important first step before consulting a healthcare provider.

Do Long Menstrual Cycles Affect Fertility?

Many people with long menstrual cycles ovulate later than average but still conceive without difficulty. A long cycle does not automatically mean you are infertile.
The menstrual cycle includes three phases: follicular, ovulatory, and luteal. The luteal phase—the time between ovulation and menstruation—lasts about 14 days for most individuals, whereas the follicular phase varies more widely.9
A longer follicular phase can lead to longer cycles, meaning ovulation occurs later. As long as ovulation is occurring, fertility is still possible.

Infertility is diagnosed after one year of trying to conceive without success.
Treatments for irregular ovulation include ovulation-inducing medications, hormone therapy, lifestyle adjustments, and assisted reproductive technologies such as in vitro fertilization (IVF).10

If you are unsure whether you are ovulating, a provider may recommend blood tests, ovulation kits, or ultrasound monitoring.

When To Contact a Healthcare Provider

It is helpful to see a healthcare provider if your cycles suddenly become irregular, if your bleeding pattern changes significantly, or if you skip multiple periods without pregnancy. Long cycles may signal thyroid disorders, POI, eating disorders, high stress, or hormonal imbalance.1
You should seek medical care if you experience:

  • bleeding again less than three weeks after your last period

  • extremely heavy bleeding

  • severe cramps that disrupt daily activities

  • periods that become increasingly painful over time

  • cycles longer than 35 days or shorter than 21 days on a regular basis

  • any menstrual change accompanied by fever, dizziness, or pelvic pain

Bleeding soon after your period may indicate infections, fibroids, endometrial polyps, or—rarely—cancer.11
An ultrasound or blood test may be needed to identify underlying causes.