How hCG doubling time is calculated
In early pregnancy, the hormone hCG rises quickly. Doctors sometimes measure it twice, a day or two apart, to see how fast it is rising. The doubling time is the number of hours it would take the level to double at the measured rate:
Doubling time = hours between tests × ln 2 ÷ ln(second result ÷ first result)
Example: 120 mIU/mL rising to 260 mIU/mL over 48 hours gives 48 × 0.693 ÷ ln(2.17) ≈ 43 hours. The calculator also shows the percentage rise, and what it would be over 24 and 48 hours.
What do the numbers mean?
In many early pregnancies, hCG is often described as doubling roughly every 48 to 72 hours, with the rise slowing as levels get higher. But healthy pregnancies vary a lot, a slower rise does not always mean a problem, and a single hCG level says little on its own. Doctors interpret hCG together with ultrasound findings and symptoms — so please discuss your results with your healthcare professional rather than relying on a doubling time.
hCG levels and twins
Twin pregnancies often have higher hCG levels on average, but the ranges overlap widely with single pregnancies, so hCG cannot reliably tell you whether you are having twins. An ultrasound is needed for that.