Placenta Previa: This condition, where the placenta partially or completely covers the internal cervical os, is a common cause of painless vaginal bleeding in the second trimester. The bleeding is typically bright red and can be heavy.

Placental Abruption: Although more commonly associated with painful vaginal bleeding, placental abruption can occasionally present with painless bleeding, especially if the abruption is marginal or partial.
Uterine Rupture: Though rare in the absence of previous uterine surgery, it can cause asymptomatic vaginal bleeding if the rupture is contained within the uterine wall or if there's minimal disruption of the uterine cavity.

Vasa Previa: A condition where fetal blood vessels are present in the membranes covering the internal cervical os, making them susceptible to rupture and severe fetal hemorrhage. It's crucial to diagnose this condition to prevent significant fetal morbidity and mortality.
Uterine Dehiscence: Similar to uterine rupture but involves a partial separation of a previous uterine scar. It can present with minimal symptoms, including asymptomatic vaginal bleeding, and requires prompt diagnosis to prevent complications.

Choriocarcinoma: A rare malignant tumor that can cause vaginal bleeding during pregnancy. It's more commonly associated with gestational trophoblastic disease and can present with systemic symptoms in addition to bleeding.
Uterine Arteriovenous Malformation (AVM): A rare vascular anomaly that can cause vaginal bleeding due to the abnormal connection between arteries and veins within the uterus. Diagnosis often requires imaging studies like ultrasound or MRI.