What Could Cause a Sudden Change in Menstrual Cycle Length?

A menstrual cycle lasts from the first day of menstruation (period bleeding) to the first day of your next period. The average menstrual cycle is 28 days. Preteens' and teens' menstrual cycle length ranges between 21 to 45 days (they may even skip periods, especially in the first two years after starting their period). Sudden changes in menstrual cycle length are not "normal" and may indicate an underlying condition or hormonal changes.

This article reviews the possible causes of menstrual cycle change, how to track changes, and next steps.

Overhead image of a woman sitting on a sofa using a menstrual cycle calendar

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Risk Factors for Sudden Menstrual Cycle Length Changes

A sudden change in cycle length (by more than a day or two) is not normal. Various factors could cause a sudden change, including:

  • Stress
  • Diet
  • Smoking
  • Exercise
  • Health conditions
  • Hormonal birth control 
  • Hormonal shifts (puberty, pregnancy, perimenopause)

What Causes Sudden Changes in Menstrual Cycle Length?

Periods are considered irregular in people with a predictable menstrual cycle under the following circumstances:

  • Cycle length is less than 21 days or more than 35 days. 
  • There are sudden variations in cycle length (more than one or two days). 
  • Bleeding doesn’t start within five days of when you expect it (late period). 
  • You don’t have menstrual bleeding for more than six weeks (missed period).
  • Cramping or bleeding interferes with daily life.

The following circumstances and conditions can cause these irregularities.


Puberty is when a person's body begins to mature. Hormonal shifts during this time trigger a menstrual cycle in people assigned female at birth.

When you start having your period, your cycle length can be between 21 and 45 days. Skipping periods for the first couple of years of starting is also common. 

If irregularities last more than a couple of years, or if you have long, heavy, painful periods, consult a healthcare provider.

Can You Get Pregnant With Irregular Cycles?

If you are sexually active, you can still get pregnant, even with irregular cycles. Consult a healthcare provider about birth control options if you are sexually active or plan to be.


Perimenopause is the time leading to menopause (when you stop having periods for 12 consecutive months). It typically begins in your mid to late 40s and lasts several years. 

The first sign of perimenopause is irregular periods, which may include:

  • Varying time between periods 
  • Skipping periods
  • Heavier or lighter blood flow or cramping

Other early signs of perimenopause include:

  • Hot flashes
  • Sleep disturbances 
  • Mood changes
  • Vaginal dryness
  • Lower libido (sex drive)

Perimenopause does not require treatment unless your symptoms are bothersome.

Lifestyle Factors

The following lifestyle factors can cause changes to an otherwise predictable menstrual cycle:

  • Stress: Try to practice stress management and relaxation techniques.
  • Smoking cigarettes: Consider smoking cessation.
  • Unhealthy diet: Eating a balanced, healthy diet may be what your body needs to return to your regular cycle. 
  • Extreme exercise: Overexercising can cause your periods to vary or stop. Consider decreasing time or intensity and see if that regulates your cycle. 
  • Weight: This includes being underweight or overweight. Consider consulting a healthcare provider or dietitian if you have difficulty managing your weight.

Hormonal Birth Control 

Hormonal birth control includes oral contraceptives (the pill), patches, injections, and implantable devices (IUDs). 

Starting, stopping, or changing hormonal birth control can affect cycle length and cause breakthrough bleeding (bleeding between periods) or amenorrhea (absence of a period). This includes minor adjustments like a change in dosing, brands, or a switch to generic. 

Birth Control Period Bleeding

Bleeding that occurs when you are on hormonal birth control differs from a true period because hormonal contraceptives prevent ovulation. It is withdrawal bleeding due to synthetic hormone variations.

Consult a healthcare provider if:

  • Cycle variations don’t even out within a few months.
  • Cramping, bleeding, or other symptoms interfere with daily life.
  • You have questions about nonhormonal birth control methods (diaphragm, sponge, condoms, sponge, spermicides, or Phexxi) during the adjustment.


Pregnancy typically causes you to miss your period. Variations in menstrual cycles can also occur due to hormonal shifts after delivery or pregnancy loss.

Pregnancy can also cause implantation bleeding when the embryo first burrows into the uterus. This type of bleeding usually occurs about 10 to 14 days after conception.

Implantation bleeding is typically lighter than menstrual bleeding. Some women mistake it for their period because it often occurs around the same time. 

If you think you may be pregnant, consider a home pregnancy test and consult a healthcare provider if it is positive. 


Polycystic ovary syndrome (PCOS) causes the ovaries to produce extra androgens (a sex hormone more abundant in males than females). One of the most common symptoms is absent periods or shorter or longer than average cycles. 

Females with PCOS may only have six to eight periods per year and may also experience the following: 

  • Weight gain 
  • Infertility 
  • Acne
  • Hair thinning 
  • Excess body and facial hair
  • Sleep apnea
  • Mental health disorders

The following are types of treatments healthcare providers may consider for PCOS.

  • Hormonal birth control
  • Weight loss assistance (women with excess weight and PCOS have more regular periods when they lose 5% to 10% of their body weight)
  • Glucophage (metformin)
  • Infertility treatment


Endometriosis is when endometrial-like tissue grows outside of the uterus (womb) and can cause:

  • Changes in menstrual cycle length and bleeding times
  • Excessive cramping
  • Pelvic (between the hips) pain
  • Pain after sex
  • Painful urination (peeing) or bowel movements (pooping)
  • Infertility 

The first line of treatment for PCOS is typically hormonal birth control and nonsteroidal anti-inflammatory drugs (NSAIDs), such as Advil or Motrin (ibuprofen).

Endometrial Hyperplasia 

Endometrial hyperplasia is when the uterine (womb) lining thickens abnormally. It’s due to excess estrogen and a lack of progesterone. It can cause:

  • Menstrual cycles that are shorter than 21 days
  • Heavy bleeding 
  • Breakthrough bleeding
  • Bleeding after menopause

Possible underlying causes include:

  • Nolvadex (tamoxifen)
  • Estrogen therapy
  • PCOS
  • Having obesity
  • Perimenopause or menopause

Healthcare providers typically treat endometrial hyperplasia with progestin, a type of synthetic progesterone in a pill, shot, vaginal cream, or intrauterine device (IUD).

Uterine Fibroids

Uterine fibroids are benign (noncancerous) growths in the uterus. They can cause:

  • Heavy bleeding or cramping
  • Shorter menstrual cycles
  • Longer bleeding times
  • Breakthrough bleeding
  • Painful sex
  • Anemia (low red blood cells)
  • Infertility
  • Pelvic pain

While not all fibroids require treatment, it is available if your symptoms disrupt daily life and may include the following: 

  • Hormonal birth control or progestin
  • Gonadotropin-releasing hormone (GnRH) agonists
  • Cyklokapron (tranexamic acid)
  • Surgery
  • Radiofrequency ablation

Ovarian Cysts

Ovarian cysts are fluid-filled sacs that grow on the ovaries. Most never cause symptoms and resolve on their own. But, some cause the following: 

  • Heavy or irregular periods 
  • Breakthrough bleeding
  • Pelvic pain
  • Painful urination (peeing)
  • Constipation or abdominal fullness

Healthcare providers often take a wait-and-watch approach with ovarian cysts, but it depends on the type and your symptoms. 

Pelvic Inflammatory Disease PID?

Pelvic inflammatory disease (PID) is an infection in the female reproductive organs. It typically occurs due to a sexually transmitted infection (STI)

Many women are unaware they have PID because it may not initially cause symptoms. However, PID can lead to the following:

  • Breakthrough bleeding
  • Heavier periods
  • Pelvic pain
  • Foul-smelling vaginal discharge
  • Fever
  • Painful sex
  • Urinary discomfort (similar to having a urinary tract infection)

Treatment for PID involves antibiotics. It’s essential to seek treatment quickly as PID can lead to complications such as scarring, infertility, and ectopic pregnancy (pregnancy outside of the uterus). 

Endocrine Disorders

Endocrine disorders disrupt hormone function and may cause premature ovulation, anovulation (no egg release during ovulation), or changes in menstrual cycle length. 

The endocrine system includes the pituitary, thyroid, pancreas, adrenal glands, and ovaries. Example disorders include:

Endocrine disorders typically require medical intervention. If your provider suspects you have one of these disorders, they may send you to an endocrinology provider specializing in hormones.

How to Track Menstrual Cycle Changes

It’s important to track your period so you will know what is normal for you. This will make you aware of when you ovulate, when to expect your period, and if something is off.

You can use a calendar, log book, or a period-tracking app to record:

  • The day you start and end your period 
  • Bleeding amounts (how often you change your pad or tampon, and if you have clots)
  • Cramping severity 
  • Premenstrual syndrome (PMS) symptoms (e.g., mood changes, sleep problems, food cravings, poor concentration)
  • Additional symptoms (e.g., painful bowel movements
  • Ovulation signs (when trying to get pregnant or avoid pregnancy)


Sudden shifts in cycle length could be from stress, overexercising, or dietary changes. Other causes include puberty, pregnancy, perimenopause, hormonal birth control, and health conditions. The average menstrual cycle (time between bleeding) is 28 days. It is “irregular” when less than 21 or more than 35 days. Bleeding times average three to seven days. 

When you don’t have a period five days past the expected date, it is considered late . Not having a period for six weeks is considered a missed period. It’s important to track your menstrual period to know when something is different for you. 

21 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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By Brandi Jones, MSN-ED RN-BC
Brandi is a nurse and the owner of Brandi Jones LLC. She specializes in health and wellness writing including blogs, articles, and education.