A 30-week pregnant patient has sudden painless vaginal bleeding. What is the likely diagnosis?

In this scenario, the sudden painless vaginal bleeding in a 30-week pregnant patient is indicative of placenta previa. This condition occurs when the placenta is abnormally positioned low in the uterus, covering part or all of the cervix. As the uterus expands during pregnancy, it can lead to bleeding, particularly in the third trimester, which aligns with the patient's current stage of pregnancy.

The bleeding associated with placenta previa is often characterized as painless, which is a key feature that helps distinguish it from other causes of bleeding during pregnancy. In contrast, abruptio placentae typically presents with painful bleeding due to the premature separation of the placenta from the uterine wall. Uterine rupture usually involves severe abdominal pain and significant fetal distress, and disseminated intravascular coagulation (DIC) would likely present with additional systemic symptoms or signs of coagulopathy.

Given the presentation of sudden and painless vaginal bleeding, the diagnosis of placenta previa aligns well with the clinical picture of the patient, making it the most probable condition in this situation.