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?

Nausea, fatigue, breast tenderness, and urinary frequency are all common symptoms of pregnancy, but their presence cannot definitively make the diagnosis of pregnancy because they are nonspecific symptoms that are not consistently found in early pregnancy. On physical exam, the pregnant uterus enlarges and becomes boggier and softer, but these changes are not usually apparent until after 6 weeks gestational age. In addition, other conditions such as adenomyosis, fibroids, or previous pregnancies can result in an enlarged uterus being palpated on physical exam. Abdominal ultrasound will not demonstrate a gestational sac until a gestational age of 5 to 6 weeks is reached. A Doppler instrument will detect fetal cardiac action usually no sooner than 10 weeks. A sensitive serum pregnancy test can detect placental HCG levels by 8 to 9 days post-ovulation, and it is therefore the most sensitive modality for detecting and diagnosing pregnancy.