Endometrial: Endometrial causes of abnormal bleeding include infection of the endometrium, endometritis, which may occur after a miscarriage (spontaneous abortion) or a delivery, or may be related to a sexually-transmitted infection of the uterus, fallopian tubes or pelvis generally termed pelvic inflammatory disease (PID). Other endometrial causes of abnormal bleeding may relate to the ways that the endometrium heals itself or develops blood vessels. Iatrogenic (caused by medical treatment or procedures): The most common Iatrogenic cause of abnormal bleeding relates to treatment with hormonal medications such as birth control pills, patches, rings, injections, implants, and intrauterine devices (IUDs). Hormone therapy for treatment of menopausal symptoms can also cause abnormal bleeding. Unscheduled bleeding that occurs during such hormonal treatment is termed "breakthrough bleeding" (BTB)  Breakthrough bleeding may result from inconsistent use of hormonal treatment, although in the initial months after initiation of a method, it may occur even with perfect use, and may ultimately affect adherence to the medication regimen. The risk of breakthrough bleeding with oral contraceptives is greater if pills are missed. Not classified: The Not Classified category of the PALM-COEIN system includes conditions that may be rare, or whose contribution to abnormal bleeding has not been well established or understood. Pregnancy
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 complications to the placenta, such as placental abruption and placenta previa.
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. 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. 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.