A 25-yrs-old woman in her 15 week of pregnancy presents with uterine bleeding and passage of a small amount of watery fluid and tissue. She is found to have a uterus that is much larger than estimated by her gestational dates. Her uterus is found to be filled with cystic, avascular, grapelike structures that do not penetrate that uterine wall. No fetal pas are fo Most likely diagnosis?