Final answer:
The most appropriate next step for a pregnant woman presenting with vaginal bleeding and stable vitals, with no pain or abdominal tenderness, is to perform a pelvic ultrasound to assess the source of bleeding.
Step-by-step explanation:
A 19-year-old G1P0 woman presents to the hospital at 25 weeks gestation with vaginal bleeding but otherwise has an uncomplicated prenatal course and shows no signs of distress or pain. Considering her situation and vital signs are stable, along with the absence of pain or tenderness in her abdomen and uterus, the most appropriate next step in management is a pelvic ultrasound. A digital cervical examination is avoided initially as it could potentially cause further bleeding if a placenta previa or low-lying placenta is present. A biophysical profile is typically not the immediate next step for assessing acute vaginal bleeding. Blood transfusion is not indicated in the absence of significant blood loss or hemodynamic instability, and while bed rest might be advised later, it is not a diagnostic measure.