Threatened abortion: This is the most common cause of vaginal bleeding in the first trimester, often due to implantation of the embryo, hormonal changes, or cervical pathology. It is characterized by bleeding with or without cramping, and the cervix is closed.

Miscarriage (spontaneous abortion): This can be either incomplete, complete, or missed, and is a common cause of bleeding in early pregnancy. Symptoms include heavy bleeding, cramping, and passage of tissue.
Ectopic pregnancy: Although less common, ectopic pregnancy is a significant cause of first-trimester bleeding, especially if associated with abdominal pain and missed periods. It occurs when the embryo implants outside the uterus.
Molar pregnancy: A rare condition where the placenta becomes abnormal, leading to bleeding, and often accompanied by severe nausea and vomiting.

Placenta previa: A condition where the placenta covers the cervix, which can cause severe bleeding, often painless. It's crucial to diagnose this condition to avoid severe hemorrhage.
Uterine rupture: Although rare in the first trimester, uterine rupture can occur, especially in women with a history of uterine surgery. It presents with severe abdominal pain and bleeding.
Ovarian torsion: This is a rare but emergency condition that can cause abdominal pain and sometimes vaginal bleeding if associated with an ovarian cyst.

Cervical polyps or cancer: These can cause irregular vaginal bleeding, including during pregnancy, though they are less common causes in the first trimester.
Vaginal or vulvar trauma: Direct injury to the vagina or vulva can cause bleeding and should be considered, especially with a history of trauma.
Coagulopathy: Bleeding disorders can exacerbate or cause vaginal bleeding during pregnancy, though they are rare and often have other associated symptoms or a known history.