A 28-year-old woman at 8 weeks' gestation presents to the emergency department with vaginal bleeding and mild lower abdominal pain. She has had no previous ultrasound in this pregnancy. On transvaginal ultrasound there is no intrauterine gestational sac and a small amount of free fluid in the pelvis. Serum β-hCG is 2500 IU/L. What is the most likely diagnosis?