A 32-year-old woman with a history of pelvic inflammatory disease presents with amenorrhea and mild lower abdominal pain. Serum β-hCG is 1200 mIU/mL. Transvaginal ultrasound shows no intrauterine gestational sac, but a 2 cm echoic mass is seen in the right fallopian tube with a live fetal pole and cardiac activity. Which feature best distinguishes a tubal ectopic pregnancy from a cornual ectopic pregnancy?