An irregular menstruation is an abnormal variation in menstrual cycle length of more than approximately eight days for a woman. A female usually experiences five or more menstrual cycles a year are five or more days shorter or longer than the length of the average cycle. Lengths ranging between eight and 20 days are considered moderately irregular. 

Alternatively, a single menstruation period may be defined as irregular if it is shorter than 21 days, or longer than 36 days. However, if they are regularly shorter than 21 days, or longer than 36 (or 35) days, the condition would rather be termed polymenorrhea (cycles with intervals of 21 days or fewer) or oligomenorrhea (infrequent, often light menstrual periods, longer than 35 days), respectively.

Very similar symptom called metrorrhagia, which generally refers to vaginal bleeding that occurs between the expected menstrual periods. The distinction between irregular cycle lengths and metrorrhagia is not always clear, and may depend on whether the bleeding is regarded as marking the menstrual period (favoring the term "irregular cycles") or being separate from it (favoring the term "metrorrhagia").

Irregular menstruation is one of the positive clinical symptoms which predict dysfunction of the hypothalamic-pituitary-ovarian axis. In order to produce a period, women body makes hormones, like estrogen and progesterone. These hormones are kept in the hypothalamus, pituitary gland, and ovaries inside women body. In order to trigger ovulation and menstruation, these parts of the body need to send signals to one another. Sometimes, these signals get crossed or skipped, causing irregular periods.
There are actually a number of things that can easily cause irregularity in hormone levels: pregnancy, stress, diet, exercise, menarche, menopause, and hormonal birth control. 


Adenomyosis

In adenomyosis, basal endometrium penetrates into hyperplastic myometrial fibers. Therefore, unlike functional layer, basal layer does not undergo typical cyclic changes with menstrual cycle and menstrual cycle occurs irregularly.

Anorexia nervosa

Anorexia nervosa, often referred to simply as anorexia, is an eating disorder characterized by a low weight, fear of gaining weight, a strong desire to be thin, and food restriction. Many people with anorexia see themselves as overweight even though they are in fact underweight. Hormone levels are affected by poor diet, thus irregular or absent menstruation is frequent symptom. 

Anovulation

An anovulatory cycle is a menstrual cycle during which the ovaries do not release an oocyte. Therefore, ovulation does not take place. Infrequent and light menstruation occurs in about 40% of women with ovulatory dysfunction. Another potential symptom is irregular menstruation. 

Asherman’s syndrome

Asherman's syndrome is a condition characterized by adhesions and/or fibrosis of the endometrium particularly but can also affect the myometrium. It is often associated with dilation and curettage of the intrauterine cavity. Often, patients experience secondary menstrual irregularities characterized by a decrease in flow and duration of bleeding (amenorrhea, hypomenorrhea, or oligomenorrhea) and become infertile. Menstrual anomalies are often but not always correlated with severity: adhesions restricted to only the cervix or lower uterus may block menstruation.

Bicornuate uterus

A bicornuate uterus, commonly referred to as a "heart-shaped" uterus, is a uterus malformation when uterus is composed of two "horns" separated by a septum. These uterine anatomy anomalies may cause irregularities or absence of menstruation, recurrent miscarriage and infertility.

Endometrial cancer

Endometrial cancer is a cancer that arises from the endometrium (the lining of the uterus or womb). It is the result of the abnormal growth of cells that have the ability to invade or spread to other parts of the body. The first sign is most often vaginal bleeding not associated with a menstrual period. Abnormal menstrual cycles or extremely long, heavy, or frequent episodes of bleeding in women before menopause may also be a sign of endometrial cancer.

Endometrial polyp

Endometrial polyps are a mass in the inner lining of the uterus. They often cause no symptoms. Where they occur, symptoms include irregular menstrual bleeding, bleeding between menstrual periods, excessively heavy menstrual bleeding (menorrhagia), and vaginal bleeding after menopause. Bleeding from the blood vessels of the polyp contributes to an increase of blood loss during menstruation and blood "spotting" between menstrual periods, or after menopause. If the polyp protrudes through the cervix into the vagina, pain (dysmenorrhea) may result.

Fibroids

Fibroids are benign tumors originating from uterine tissue. Symptoms depend on the location and size of the fibroid. Important symptoms related to menstruation include abnormal uterine bleeding and heavy or painful periods.

Hyperprolactinemia

In women, a high blood level of prolactin often causes hypoestrogenism with anovulatory infertility and a decrease in menstruation. In some women, menstruation may disappear altogether (amenorrhoea). In others, menstruation may become irregular or menstrual flow may change.

Menopause

Menopause, also known as the climacteric, is the time in most women's lives when menstrual periods stop permanently, and they are no longer able to have children. Menopause typically occurs between 45 and 55 years of age. Medical professionals often define menopause as having occurred when a woman has not had any vaginal bleeding for a year. It may also be defined by a decrease in hormone production by the ovaries. Before menopause, a woman's periods typically become irregular, which means that periods may be longer or shorter in duration or be lighter or heavier in the amount of flow. This irregularity is caused by estradiol levels and patterns whose production remain relatively unchanged or may increase compared to young women, but the cycles become frequently shorter or irregular.

Menstrual cycle disorders

A menstrual disorder is an abnormal condition in a woman's menstrual cycle. It is classified in several types including disorders of ovulation, cycle length, flow and pain. Irregular menstruation is a type of disorder of cycle length, but it is correlated to other possible types and could occur altogether.

Oligomenorrhea

Oligomenorrhea is infrequent (or, in occasional usage, very light) menstruation. More strictly, it is menstrual periods occurring at intervals of greater than 35 days, with only four to nine periods in a year. Also, menstrual periods should have been regularly established previously before the development of infrequent flow. Oligomenorrhea could be a result of polycystic ovary syndrome (PCOS), eating disorders or endurance exercise, which all affect the regularity of menstrual cycle. 

Ovarian cancer

Ovarian cancer is a cancer that forms in an ovary. It results in abnormal cells that have the ability to invade or spread to other parts of the body. When this process begins, there may be no or only vague symptoms. Symptoms become more noticeable as the cancer progresses. Irregular menstruation is one of the most typical symptoms. 

Pelvic inflammatory disease

Pelvic inflammatory disease or pelvic inflammatory disorder (PID) is an infection of the upper part of the female reproductive system namely the uterus, fallopian tubes, and ovaries, and inside of the pelvis. Often there may be no symptoms. Signs and symptoms, when present may include lower abdominal pain, vaginal discharge, fever, burning with urination, pain with sex, or irregular menstruation.

Polycystic ovary syndrome (PCOS), ovarian cysts

In women with PCOS, the ovary doesn’t work physiological and the body is under hormonal imbalance. The follicles may grow and fill up with fluid but ovulation doesn’t occur due to absent of progesterone production. Without ovulation those follicles may persist as ovarian cysts and menstrual cycle is irregular or absent. Additionally, those ovaries may produce higher count of men hormones which also act as prevention to ovulation. 

Primary ovarian insufficiency

Due to hormone imbalance, there is irregular or missed menstrual periods. 

Thyroid disorders

Women with hyper or hypo-thyroidism sometimes have ovulation problems. Thyroid dysfunction can halt ovulation by upsetting the balance of the body’s natural reproductive hormones. Hypothyroidism is associated with delay in the onset of puberty, anovulation, amenorrhea or hypermenorrhea, menstrual irregularity, infertility and increased frequency of spontaneous abortions. It was suggested that these alterations may be caused by a decrease in luteinizing hormone (LH) secretion.



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Sources

Anorexia nervosa ―sourced from Wikipedia licensed under CC BY-SA 3.0
Polycystic ovary syndrome ―sourced from Wikipedia licensed under CC BY-SA 3.0
Endometriosis ―sourced from Wikipedia licensed under CC BY-SA 3.0
Bicornuate uterus ―sourced from Wikipedia licensed under CC BY-SA 3.0
Anovulation ―sourced from Wikipedia licensed under CC BY-SA 3.0
Ovarian cancer ―sourced from Wikipedia licensed under CC BY-SA 3.0
Pelvic inflammatory disease ―sourced from Wikipedia licensed under CC BY-SA 3.0
Menstrual disorder ―sourced from Wikipedia licensed under CC BY-SA 3.0
Endometrial polyp ―sourced from Wikipedia licensed under CC BY-SA 3.0
Uterine_fibroids ―by Hic et nunc licensed under CC BY-SA 3.0
Endometrial cancer ―sourced from Wikipedia licensed under CC BY-SA 3.0
Asherman's syndrome ―sourced from Wikipedia licensed under CC BY-SA 3.0
Irregular menstruation ―sourced from Wikipedia licensed under CC BY-SA 3.0
Oligomenorrhea ―sourced from Wikipedia licensed under CC BY-SA 3.0
Adenomyosis ―sourced from Wikipedia licensed under CC BY-SA 3.0
Irregular Periods ―by aditigupta licensed under CC BY-SA 3.0
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