hCG

From syncytiotrophoblast - first detectable at 10 days, peaks at 10 wks, plateaus at 20.

alpha subunit like: LSH, FSH, TSH
beta subunit: unique

high levels: twins, hytadifiform mole, choriocarcinoma, embyronal carcinoma
low levels: ectopic preg, abortion

hcg < 5,000 don't expect to see sac on transabd ultrasound - do transvaginal