Vaginal bleeding occurs during 15-25% of first trimester pregnancies. Of these, half go on to miscarry and half bring the fetus to term. There are a number of causes including rupture of a small vein on the outer rim of the placenta. It can also herald a miscarriage or ectopic pregnancy, which is why urgent ultrasound is required to separate the two causes. Bleeding in early pregnancy may be a sign of a threatened or incomplete miscarriage.

In the second or third trimester a placenta previa (a placenta partially or completely overlying the cervix) may bleed quite severely. Placental abruption is often associated with uterine bleeding as well as uterine pain.

Endometrial atrophy, uterine fibroids, and endometrial cancer are common causes of postmenopausal vaginal bleeding.

Breakthrough bleeding that does not resolve on its own is a common reason for women to switch to different pill formulations, or to switch to a non-hormonal method of birth control.

Breakthrough bleeding is most commonly caused by an excessively thick endometrium (uterine lining). This is not a dangerous condition, though the unpredictable and often lengthy periods of bleeding are unpleasant. Breakthrough bleeding may also be caused by hormonal effects of ovulation. Breakthrough bleeding may also itself be a symptom of pregnancy.

Breakthrough bleeding is most common when a woman first begins taking oral contraceptives, or changes from one particular oral contraceptive to another, though it is possible for breakthrough bleeding to happen at any time. Smokers are especially prone to breakthrough bleeding while taking oral contraceptives; though many users experience breakthrough bleeding in the first three cycles of taking the pill, non-smokers tend to see the bleeding dissipate more quickly than smokers.

Breakthrough bleeding is likely due to hormonal fluctuations. The body is programmed to make certain estrogen levels each day and the estrogen (and some additional hormones, such as FSH, LH, and Progesterone) are responsible for regulating endometrium shedding. Therefore, when new levels of hormones enter the body through oral contraceptives, the body is provided with two ways to receive estrogen. These excess estrogen levels can cause pre-period bleeding (bleeding through). This should be regulated in several months.

According to "Lange Gynecology and Obstetrics", 8th edition, the most common side effect associated with OC use is breakthrough bleeding. It usually occurs during the first one or two cycles and resolves itself spontaneously. Another common problem is amenorrhea. Persistent break through bleeding and amenorrhea commonly reflect an atrophic, or thin and poorly developed, endometrium.

Use of combined estrogen and progesterone eliminates the normal endogenous hormonal cycling and gradually produces atrophy of the endometrial glands. This is because the dosage of estrogen in the OCs pills is much lower than the quantity produced naturally by the ovaries. Higher quantities produced by the ovaries induce proliferation, but low levels supplied by the pills produce atrophy but are sufficient to inhibit the endogenous secretion of the gonadotropins.

The exact chain of events that lead from an atrophic endometrium to the spotting between menses is not explained by the text. This condition may be corrected by using a pill with a higher estrogen (which will stimulate further proliferation of the endometrium) or lower progestin content (which will reduce its stability).

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.

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.

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.

It can occur due to any of several causes, including hormonal imbalance, endometriosis, uterine fibroids, usage of progestin-only contraception, or cancer. It can lead to anemia in long-standing cases.

A menstrual disorder is an abnormal condition in a woman's menstrual cycle.

Irregular menstruation is a menstrual disorder whose manifestations include irregular cycle lengths as well as metrorrhagia (vaginal bleeding between expected periods).

"Polymenorrhea" is the medical term for cycles with intervals of 21 days or fewer.

"Irregular menstruation" is where there is variation in menstrual cycle length of more than approximately eight days for a woman. The term "metrorrhagia" is often used for irregular menstruation that occurs between the expected menstrual periods.

"Oligomenorrhea" is the medical term for infrequent, often light menstrual periods (intervals exceeding 35 days).

"Amenorrhea" is the absence of a menstrual period in a woman of reproductive age. Physiologic states of amenorrhoea are seen during pregnancy and lactation (breastfeeding). Outside of the reproductive years there is absence of menses during childhood and after menopause.

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.