Pathophysiology
Clinical meaning
Human chorionic gonadotropin (hCG) is a glycoprotein hormone produced by syncytiotrophoblast cells shortly after implantation, detectable in serum approximately 8-11 days post-conception. In a normal intrauterine pregnancy (IUP), hCG doubles approximately every 48-72 hours during the first 8-10 weeks, reaching peak levels of 50,000-200,000 mIU/mL by weeks 10-12, then declining and plateauing. The discriminatory zone is the hCG threshold above which a viable intrauterine pregnancy should be visualized on transvaginal ultrasound (TVUS), typically 1,500-3,500 mIU/mL depending on institutional standards. When hCG exceeds the discriminatory zone but no intrauterine gestational sac is seen, the differential includes ectopic pregnancy, recent complete miscarriage, or very early pregnancy not yet visible. The NP applies serial hCG trending, discriminatory zone analysis, TVUS correlation, and clinical assessment to construct a diagnostic algorithm. An hCG rise of less than 53% over 48 hours suggests an abnormal pregnancy (ectopic or nonviable IUP) but cannot definitively differentiate between the two. A declining hCG suggests miscarriage but does not exclude ectopic pregnancy, as 20-30% of ectopic pregnancies present with falling hCG. The 'pregnancy of unknown location' (PUL) framework is applied...