hCG (human chorionic gonadotropin)
hCG is the hormone produced after implantation and measured by pregnancy tests. In early pregnancy its level rises quickly, and clinicians look at the rate of rise between two measurements rather than at any single value.
Example
A rise from 100 to 200 mIU/mL over 48 hours is a doubling time of exactly 48 hours. What a given rate means requires ultrasound and history alongside it, which is a clinician's job rather than a calculator's.