Vaginal bleeding in the third trimester of pregnancy is most likely associated with either placenta previa or placental abruption but could be due to several life-threatening causes. Placenta previa occurs when the placenta attaches near or over the cervical os, while placenta abruption refers to the premature separation of placenta from the uterine wall. Both conditions can be determined through abdominal ultrasound. X-ray of the abdomen is not routinely performed on pregnant patients. A pelvic examination should not be performed until placenta previa is ruled out. Nonstress test assesses fetal reactivity, but would not be a priority in this instance.