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

The most likely diagnosis for a 30-week pregnant patient who experiences sudden onset of painless vaginal bleeding is placenta previa. This condition occurs when the placenta is abnormally positioned, covering part or all of the cervix, which can lead to painless bleeding as the uterus expands and puts pressure on the placental attachment.

In cases of placenta previa, the bleeding is typically bright red and does not cause discomfort, distinguishing it from other potential causes of bleeding during pregnancy. This scenario aligns with the presentation of the patient experiencing sudden painless vaginal bleeding at 30 weeks of gestation, which is a common gestational age for symptoms to manifest as the placenta's position changes with the growth of the uterus.

Other conditions, such as abruptio placentae and uterine rupture, typically present with painful bleeding and are associated with abdominal pain and fetal distress. Disseminated intravascular coagulation (DIC) may lead to bleeding but is usually secondary to other obstetric complications and is characterized by severe systemic symptoms rather than isolated painless vaginal bleeding. Therefore, the clinical presentation strongly supports placenta previa as the most likely diagnosis in this situation.