HCG is short for Human chorionic gonadotropin which is a specific hormone that is released into your body (blood, urine) at the time of pregnancy, it helps sustain an early pregnancy until the placenta can take over, this is what your hpt picks up on to give you a positive, there are two types of blood te4sts that measure this, one just says yes or no to pregnancy, the other is a couple of tests 2-3 days apart and this measure the amunt the hcg rises.