Systemic Causes
Cirrhosis
Coagulopathy (Von Willebrand disease, ITP)
Group A strep vaginitis (prepubertal girls)
Hormone replacement therapy
Anticoagulants
Selective Serotonin Reuptake Inhibitors (SSRIs)
Hypothyroidism
Polycystic Ovary Syndrome
Secondary anovulation
Reproductive Tract Causes
Adenomyosis
Atrophic endometrium<unk> 
Endometriosis
Fibroids
Foreign Body
Infection (vaginitis, PID)
IUD
Neoplasia (especially in women >45 years old or in younger women with other risk factors)
Vaginal Trauma
Dysfunctional uterine bleeding
Systemic Causes
Nonpregnant vaginal bleeding
Cirrhosis
Prepubertal vaginal bleeding
Coagulopathy 
Postpartum hemorrhage
Group A strep vaginitis 