Miscarriage: This is the most common cause of vaginal bleeding in the first trimester, and at 12 weeks, the risk, although decreased, still exists. Bleeding can range from light spotting to heavy bleeding, and it may be accompanied by cramping.

Threatened Abortion: This condition is characterized by vaginal bleeding with or without cramping, but the cervix remains closed. It's a common cause of bleeding in early pregnancy.
Subchorionic Hematoma: A collection of blood between the placenta and the uterus, which can cause vaginal bleeding. It's more common in the first trimester but can still occur at 12 weeks.
Cervical Lesions or Infections: Conditions like cervical polyps, cervicitis, or cervical cancer can cause vaginal bleeding. Although less common, they should be considered, especially if other symptoms are present.

Ectopic Pregnancy: Although the risk decreases as pregnancy advances, an ectopic pregnancy can still present with vaginal bleeding at 12 weeks, especially if there's a rupture. This is a medical emergency.
Placenta Previa: A condition where the placenta covers the cervical os, which can cause painless vaginal bleeding. It's crucial to diagnose this condition to avoid severe bleeding.
Molar Pregnancy: A rare condition where the placenta becomes abnormal, leading to vaginal bleeding. It's essential to diagnose this condition due to the risk of complications, including gestational trophoblastic disease.

Uterine Rupture: Extremely rare in the absence of previous uterine surgery, but it can occur and is a medical emergency.
Vaginal or Uterine Tumors: Rare causes of vaginal bleeding, but they should be considered, especially if the bleeding is persistent or if there are other symptoms like a palpable mass.
Coagulopathy: Bleeding disorders can cause vaginal bleeding during pregnancy. Although rare, they are important to diagnose due to the potential for significant bleeding complications.