Bleeding before the expected time of menarche could be a sign of precocious puberty. Other possible causes include the presence of a foreign body in the vagina, molestation, vaginal infection (vaginitis), and rarely, a tumor.

Most unusual bleeding or irregular bleeding (metrorrhagia) in premenopausal women is caused by changes in the hormonal balance of the body. These changes are not pathological. Exceptionally heavy bleeding during menstruation is termed "menorrhagia" or "hypermenorrhea", while light bleeding is called "hypomenorrhea". Women on hormonal contraceptives can experience breakthrough bleeding and/or withdrawal bleeding. Withdrawal bleeding occurs when a hormonal contraceptive or other hormonal intake is discontinued.

There are pathological causes of unusual vaginal bleeding as well. Dysfunctional uterine bleeding is a common cause of menorrhagia and irregular bleeding. It is due to a hormonal imbalance, and symptoms can be managed by use of hormonal contraception (although hormonal contraception does not treat the underlying cause of the imbalance). If it is due to polycystic ovary syndrome, weight loss may help, and infertility may respond to clomifene citrate. Uterine fibroids (leiomyoma) are benign tumors of the uterus that cause bleeding and pelvic pain in approximately 30% of affected women. Adenomyosis, a condition in which the endometrial glands grow into the uterine muscle, can cause dysmenorrhea and menorrhagia. Cervical cancer may occur at premenopausal age, and often presents with "contact bleeding" (e.g. after sexual intercourse). Uterine cancer leads to irregular and often prolonged bleeding. In recently pregnant women who have delivered or who have had a miscarriage, vaginal bleeding may be a sign of endometritis or retained products of conception.

The bleeding is usually light, often referred to as "spotting," though a few women may experience heavier bleeding.

Many women find that the breakthrough bleeding ceases after one or two cycles.

Breakthrough bleeding (BTB) is any of various forms of vaginal bleeding, usually referring to mid-cycle bleeding in users of combined oral contraceptives, as attributed to insufficient estrogens. It may also occur with other hormonal contraceptives. Sometimes, "breakthrough bleeding" is classified as "abnormal" and thereby as a form of metrorrhagia, and sometimes it is classified as "not abnormal".

Menometrorrhagia is a condition in which prolonged or excessive uterine bleeding occurs irregularly and more frequently than normal. It is thus a combination of metrorrhagia and menorrhagia.

Dysfunctional uterine bleeding (DUB) is abnormal genital tract bleeding based in the uterus and found in the absence of demonstrable structural or organic disease. It is usually due to hormonal disturbances: reduced levels of progesterone cause low levels of prostaglandin F2alpha and cause menorrhagia (abnormally heavy flow); increased levels of tissue plasminogen activator (TPA) (a fibrinolytic enzyme) lead to more fibrinolysis.

Diagnosis must be made by exclusion, since organic pathology must first be ruled out.

DUB can be classified as "ovulatory" or "anovulatory", depending on whether ovulation is occurring or not. It is usually a menstrual disorder, although abnormal bleeding from the uterus is possible outside menstruation.

Some sources state that the term "dysfunctional" implies a hormonal mechanism. Use of the term "abnormal uterine bleeding" is preferred in today's medicine.

The initial workup includes exclusion of pregnancy and cancer, by performing a pregnancy test, a pelvic exam and a gynecologic ultrasound. Further workup depends on outcomes of the preceding tests and may include hydrosonography, hysteroscopy, endometrial biopsy, and magnetic resonance imaging.

10% of cases occur in women who are ovulating, but progesterone secretion is prolonged because estrogen levels are low. This causes irregular shedding of the uterine lining and break-through bleeding. Some evidence has associated Ovulatory DUB with more fragile blood vessels in the uterus.

It may represent a possible endocrine dysfunction, resulting in menorrhagia or metrorrhagia.

Mid-cycle bleeding may indicate a transient estrogen decline, while late-cycle bleeding may indicate progesterone deficiency.

Abnormal uterine bleeding is a general category that includes any bleeding from menstrual or nonmenstrual causes. Hypomenorrhea is abnormally light menstrual periods. Menorrhagia (meno = month, rrhagia = excessive flow/discharge) is an abnormally heavy and prolonged menstrual period. Metrorrhagia is bleeding at irregular times, especially outside the expected intervals of the menstrual cycle. If there is excessive menstrual and uterine bleeding other than that caused by menstruation, menometrorrhagia (meno = prolonged, metro = uterine, rrhagia = excessive flow/discharge) may be diagnosed. Causes may be due to abnormal blood clotting, disruption of normal hormonal regulation of periods or disorders of the endometrial lining of the uterus. Depending upon the cause, it may be associated with abnormally painful periods.

Disorders of ovulation include oligoovulation and anovulation:

- Oligoovulation is infrequent or irregular ovulation (usually defined as cycles of ≥36 days or <8 cycles a year)

- Anovulation is absence of ovulation when it would be normally expected (in a post-menarchal, premenopausal woman). Anovulation usually manifests itself as irregularity of menstrual periods, that is, unpredictable variability of intervals, duration, or bleeding. Anovulation can also cause cessation of periods (secondary amenorrhea) or excessive bleeding (dysfunctional uterine bleeding).

Irregular cycles or irregular periods is an abnormal variation in length of menstrual cycles. A female usually experiences cycle length variations of up to eight days between the shortest and longest cycle lengths. Lengths ranging between eight and 20 days are considered moderately irregular. Variation of 21 days or more is considered very irregular.

Alternatively, a single menstruation period may be defined as irregular if it is shorter than 21 days or longer than 36 days. If they are regularly shorter than 21 days or longer than 36 (or 35) days, the condition is termed polymenorrhea or oligomenorrhea, respectively.

Other types of conditions that can be referred to by "irregular menstruation" include:

- 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. It 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").

- Oligomenorrhea generally refers to infrequent menstruation, More strictly, it is menstrual periods occurring at intervals of greater than 35 days, with only four to nine periods in a year. Menstrual periods should have been regularly established before the development of infrequent flow and often (but not always) involves irregular intervals. In contrast to "irregular cycles", the interval between one cycle and the next may be consistent but can be regarded as "irregular" compared to the cycle length of a female without oligomenorrhea. Women with oligomenorrhea often have irregular cycles as well.

- "Polymenorrhea" is the medical term for cycles with intervals of 21 days or fewer. It can be regarded as the opposite of oligomenorrhea.