A 34-year-old woman at 13 weeks of gestation presents with vaginal bleeding and increasing abdominal distension. The uterus is approximately 18 weeks in size, her serum β-human chorionic gonadotrophin concentration is 390 000 IU/L, and ultrasonography shows a heterogeneous intrauterine mass containing numerous small cystic spaces with no identifiable fetus. Both ovaries contain multiloculated cysts. Which one of the following is the most likely diagnosis?