Hormonal contraception is a common, pharmacologic cause of irregular menstrual bleeding Of women who use birth control pills containing 35 μg or less of estrogen, 5% to 10% will have breakthrough intermenstrual spotting or bleeding, especially during the first months of contraceptive pill use Breakthrough bleeding is also a common side effect of progestin-only contraceptive pills, injectable medroxyprogesterone, and long-acting progestin implants Many patients using birth control pills experience estrogen and progesterone withdrawal bleeding if they forget to take one or several pills Physiologic anovulatory cycles are frequent, especially in the first years after menarche, stemming from immaturity of the hypothalamic–pituitary–ovarian axis The physiology of anovulatory cycles deserves special mention as it is one of the most common causes of irregular bleeding in adolescents In the absence of ovulation, the corpus luteum never forms, and estrogen continues to act on the endometrium unopposed by progesterone The lining becomes increasingly thicker and eventually outgrows the supporting capabilities of the stroma Punctate areas of endometrial shedding give way to more significant bleeding as the deeper layers are affected and the spiral arterioles are exposed The treatment of AUB from physiologic anovulation requires the administration of both exogenous estrogen and progesterone—estrogen to stimulate endometrial regrowth in the excessively thin areas and progesterone to strengthen the stromal support Anovulatory cycles caused by polycystic ovarian syndrome should also be considered in a teenager with AUB Menstrual cycles may be infrequent and irregular, as androgenic excess contributes to abnormal ovarian function and anovulation Polycystic ovarian syndrome is common among adolescents and should be considered in adolescents with abnormal bleeding and stigmata of androgen excess (hirsutism, acne, obesity) Hypothyroidism should be considered if the patient has other symptoms or signs of thyroid dysfunction A functioning ovarian cyst is a less common cause of vaginal bleeding but should be considered especially in the teenager with AUB and an adnexal mass or tenderness Infection In the nonpregnant patient with AUB, infectious causes such as cervicitis or pelvic inflammatory disease should be considered, especially if there is pelvic pain or tenderness Abnormal bleeding occurs in nearly one-third of patients with pelvic inflammatory disease, generally as a result of endometritis Sexually transmitted infections and pelvic inflammatory disease are discussed in detail in