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The Duration of the Menstrual Cycle

There is quite a lot of variability in the menstrual cycle. The average cycle is 28 days long, but anything from 21 to 35 days is considered normal. Tracking your menstrual cycles can help you understand what is normal for you. This will also help you notice irregularities, such as a delayed or missed period, or spotting.

What is considered a normal menstrual cycle can vary greatly from person to person, just like one person’s cycle can vary from month to month. Irregularities in the menstrual cycle are common and, in most cases, no cause for concern. However, they sometimes signal health problems or significant life changes, such as pregnancy or the onset of menopause.

The menstrual cycle is a series of natural changes, governed by hormones that prepare a woman’s body for the possibility of pregnancy—the development and release of an egg and the thickening of the uterine lining to provide nutrients for the egg, should it become fertilized. If ovulation occurs and the egg is not fertilized, the uterine lining is shed through the vagina as the menstrual period, lasting two to seven days on average.

The length of the menstrual cycle is calculated from the first day of menstruation until the first day of the next menstrual period. Although 28 days is said to be the average length, a cycle of 21 to 35 days is considered to be within normal range.


Studies have found that only about 13 percent of women have a 28-day cycle.

It is normal for young women to have irregular menstrual cycles when they first begin to menstruate. Many women find that their cycles stabilize within two years, but some have irregular periods for their entire lives. It is unusual for women to experience a difference of more than 4 days between the length of their shortest and longest cycles, but a difference of up to 8 days is still considered regular. Variations of 8–20 days between shortest and longest cycles is considered irregular, but not abnormal. Variations of 21 days or longer are considered very irregular.

Reasons to be concerned: bleeding lasts for more than seven days; bleeding is heavier than usual; bleeding happens between periods; you had regular periods, but they have suddenly become irregular; you aren’t pregnant but your periods stop for more than 90 days; you develop severe pain during your period; you suddenly feel sick and feverish after using tampons.

If your menstrual cycle does not fall within the normal range, or the duration of the cycle is unstable and variable, talk to your gynecologist.

The four phases of a regular cycle

Normal menstruation is called eumenorrhea. The first phase is the menstrual phase, when your body sheds the nutrient-rich endometrium created during the previous cycle.

The next phase is the pre-ovulatory or follicular phase, spanning from the beginning of the menstrual period to ovulation. At the start of the cycle two hormones increase follicle stimulating hormone (FSH) and luteinizing hormone (LH). First the FSH stimulates 10–20 follicles or egg sacs to begin developing. Around day five LH causes them to mature and begin producing estrogen themselves. Through the interplay of LH and estrogen the follicles continue to mature. By day eight one follicle has become dominant while the rest recede and are reabsorbed. Estrogen also stimulates the creation of a new uterine lining. The follicular phase is complete around day thirteen.

This is followed by the ovulatory phase. A peak in estrogen triggers a surge in LH, which in turn triggers ovulation within 24-48 hours. Once an egg is released into the uterus it is viable for a day, while sperm is viable for three to four days. If a woman has a regular cycle and her body is functioning normally, this is the fertile window when a woman can get pregnant.

The final phase of the cycle is the luteal phase. During this phase the stimulated follicles have been reabsorbed into the body and estrogen levels decline. The mature follicle that released its egg is now a corpus luteum and produces another hormone—progesterone. Progesterone helps ‘feed’ and maintain the uterine lining should the egg be fertilized and implant into the uterine wall. If this does not happen, the lining is shed, and the process begins again.


When the cycle is too short

A shortened cycle (less than 21 days) is called polymenorrhea. A short cycle may be due to a shortening of either the follicular (pre-ovulatory) or luteal (post-ovulatory) phase.

A short follicular phase, or early ovulation is one of the most common causes of female infertility. Stress or a significant change in daily life such as marriage, death, loss or change of job, or moving can influence a woman’s menstrual cycle and often are the cause of irregularities women experience.

Early ovulation may indicate poor egg quality or follicle function. Among the possible causes are polycystic ovary syndrome (PCOS)—a hormonal disorder provoking the ovaries to develop numerous small collections of fluid (follicles) and fail to regularly release eggs, hyperprolactinemia—an excess of the hormone prolactin in a person’s body, a fibroma or benign tumor, an ovarian abnormality, or a disorder of the hypothalamic-pituitary complex (where the hormones regulating the cycle are produced).

Sometimes the luteal phase is too short for fertilization and implantation to take place. Luteal insufficiency can be due to a lack of progesterone. Progesterone is essential for the development of a nutrient-rich endometrium which allows for proper implantation, which usually takes place between the 6th and the 10th day after fertilization. If the luteal phase is too short, the egg does not have time to migrate to the uterus and implant in the uterine wall before the start of the new cycle.

Hormonal contraceptives can help lengthen the luteal phase, but they also stop ovulation from occurring. This means they aren’t helpful for women who are trying to get pregnant. However, a doctor can prescribe other medications to help women with short luteal phases conceive.

Other possible reasons for a shorter menstrual cycle:

  • Natural hormonal changes related to puberty or menopause.
  • Endometriosis—when uterine-like tissue grows outside the womb. Endometriosis is often painful and can also cause abnormalities of the menstrual cycle, such as shorter cycles or bleeding between periods.
  • An STD—gonorrhea and chlamydia can shorten the length of a cycle, and also cause unusual bleeding between periods.

When the cycle is too long

Infrequent menstrual periods and/or a very small amount of menstrual bleeding is called oligomenorrhea. Some variation in menstruation is normal, but a woman of childbearing age who regularly goes more than 35 days without menstruating may be diagnosed with oligomenorrhea.


Sometimes this condition can lead to amenorrhea—the absence of menstruation.

Oligomenorrhea is often a side effect of hormonal contraception. Some women experience increasingly lighter periods for three to six months after they begin using birth control. Sometimes, their periods stop completely. The opposite is also true—if you have just stopped using a hormonal contraceptive or had your IUD removed, your cycle may be longer for a few months.

Other causes include natural adaptation at puberty and perimenopause, strenuous exercise, obesity, eating disorders, such as anorexia nervosa and bulimia, diabetes or thyroid problems, and polycystic ovary syndrome (PCOS).

If you have not recently given birth, are not breastfeeding, or have not recently stopped using a hormonal contraceptive but your cycles are still more than 35–40 days long, see your gynecologist to rule out a more serious underlying medical condition. Remember that your physical and mental health have an impact on your menstrual cycle - taking care of yourself is the best preemptive strategy for battling irregular cycles.

You can track your period using WomanLog. Download WomanLog now:

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