19 year old primigravida, 12 weeks POG, c/o excessive nausea & vomiting & irregular vaginal bleeding for past 1 week. UPT positive at home. PR 100/min & BP is 110/70 mm Hg, cardioresp exam is normal & no c/o breathlessness or tremors. P/A exam - uterus enlarged to 20 weeks size, fetal parts not palpable, FHS not heard. P/S exam - os closed, no active bleeding. What is the most likely diagnosis ?