A patient comes to your office with LMP 4 weeks ago. She denies any symptoms such as nausea, fatigue, urinary frequency, or breast tenderness. She thinks that she may be pregnant because she has not gotten her period yet and is very anxious to find out because she has a history of a previous ectopic pregnancy and wants to be sure to get early prenatal care. Which of the following evaluation methods is most sensitive in diagnosing pregnancy?