Placenta Previa: This condition, where the placenta partially or completely covers the internal cervical os, is a common cause of vaginal bleeding in the second and third trimesters. The risk increases with gestational age, and it's a significant concern at 28 weeks.

Placental Abruption: Also known as placental abruption, this condition involves the premature separation of the placenta from the uterus. It can cause vaginal bleeding and is a significant risk factor for maternal and fetal morbidity.
Cervical Insufficiency: Weakening of the cervix can lead to preterm labor and vaginal bleeding. It might be suspected if there's a history of cervical surgery or previous preterm births.
Uterine Rupture: Although less common, uterine rupture can occur, especially in women with a history of uterine surgery (like a previous cesarean section). It's a medical emergency.

Uterine Dehiscence: Similar to uterine rupture but involves a partial tear of the uterine wall, often at the site of a previous uterine incision. It's critical to diagnose promptly to prevent progression to a complete rupture.
Vasa Previa: A condition where fetal blood vessels are exposed in the lower uterine segment and are at risk of rupture. This is a life-threatening condition for the fetus and requires immediate intervention.
Amniotic Fluid Embolism: Although rare, amniotic fluid embolism is a catastrophic condition that can occur during pregnancy, leading to severe maternal and fetal distress.

Choriocarcinoma: A rare type of cancer that can occur during pregnancy, often presenting with vaginal bleeding. It's essential to consider in the differential diagnosis, especially if other causes are ruled out.
Uterine Artery Aneurysm or Pseudoaneurysm: These are rare vascular abnormalities that can cause vaginal bleeding during pregnancy. They might be considered if other common causes are excluded and there's evidence of significant blood loss or hemodynamic instability.