You are managing a 24 year old woman in A&E who presented with severe abdominal pain and some light PV bleeding. Examination on admission revealed mild abdominal tenderness and a speculum exam showed cervical excitation and a closed os. Observations were all within normal range.

Her bloods came back showing an elevated B-HCG of 550 (everything else normal) and so you ordered an USS which showed an empty uterine cavity and no signs of any free fluid in the abdomen. You decide to keep her in for observation.

48 hours later, her repeat bloods come back showing that her B-HCG has risen to 560. What is your top differential?