Vaginal bleeding is any expulsion of blood from the vagina. This bleeding may originate from the uterus, vaginal wall, or cervix. Generally, it is either part of a normal menstrual cycle or is caused by hormonal or other problems of the reproductive system, such as abnormal uterine bleeding.

Regular monthly vaginal bleeding during the reproductive years, menstruation, is a normal physiologic process. During the reproductive years, bleeding that is excessively heavy (menorrhagia or heavy menstrual bleeding), occurs between monthly menstrual periods (intermenstrual bleeding), occurs more frequently than every 21 days (abnormal uterine bleeding), occurs too infrequently (oligomenorrhea), or occurs after vaginal intercourse (postcoital bleeding) should be evaluated.

The causes of abnormal vaginal bleeding vary by age, and such bleeding can be a sign of specific medical conditions ranging from hormone imbalances or anovulation to malignancy (cervical cancer, vaginal cancer or uterine cancer). In young children, or elderly adults with cognitive impairment, the source of bleeding may not be obvious, and may be from the urinary tract (hematuria) or the rectum rather than the vagina, although most adult women can identify the site of bleeding. When vaginal bleeding occurs in prepubertal children or in postmenopausal women, it always needs medical attention.

Vaginal bleeding during pregnancy can be normal, especially in early pregnancy. However, bleeding may also indicate a pregnancy complication that needs to be medically addressed. During pregnancy bleeding is usually, but not always, related to the pregnancy itself.

The treatment of vaginal bleeding is dependent on the specific cause, which can often be determined through a thorough history, physical, and medical testing.

The parameters for normal menstruation have been defined as a result of an international process designed to simplify terminologies and definitions for abnormalities of menstrual bleeding. The causes of abnormal vaginal bleeding vary by age.

Bleeding in children is of concern if it occurs before the expected time of menarche and in the absence of appropriate pubertal development. Bleeding before the onset of pubertal development deserves evaluation. It could result from local causes or from hormonal factors. In children, it may be challenging to determine the source of bleeding, and "vaginal" bleeding may actually arise from the bladder or urethra, or from the rectum.

Vaginal bleeding in the first week of life after birth is a common observation, and pediatricians typically discuss this with new mothers at the time of hospital discharge. During childhood, one of the most common causes of vaginal bleeding is presence of a foreign body in the vagina which may be caused by normal self-exploration or can be indicative of sexual abuse. This is often associated with pelvic pain, foul discharge, or recurrent genitourinary infections. Other causes include trauma (either accidental or non accidental, i.e. child sexual abuse or molestation), urethral prolapse, vaginal infection (vaginitis), vulvar ulcers, vulvar skin conditions such as lichen sclerosus, and rarely, a tumor (benign or malignant vaginal tumors, or hormone-producing ovarian tumors). Hormonal causes include central precocious puberty, or peripheral precocious puberty (McCune–Albright syndrome), or primary hypothyroidism.