About

In early viable intrauterine pregnancies, beta-hCG levels rise in a predictable curvilinear fashion. The 'Doubling Time'-the number of hours it takes for the hormone concentration to increase two-fold-is the primary metric physicians use to assess the health of a pregnancy before ultrasound findings are definitive. A doubling time that slows significantly or plateaus early may indicate an ectopic pregnancy or impending miscarriage.

This calculator accepts exact dates and times for two sequential blood tests to compute the rate of rise with hourly precision. While the standard rule of thumb is "doubles every 48 hours," normal ranges actually vary by initial concentration: faster (under 48h) for low levels, and slower (up to 96h) as levels exceed 6,000 mIU/mL.

Formulas

The doubling time T₂ is derived from the exponential growth equation. Given two measurements C₁ and C₂ taken h hours apart:

T₂ = h ⋅ log(2)log(C₂ / C₁)

If the result is negative, it indicates a halving time (decreasing levels), which is tracked during miscarriage resolution.

Reference Data

hCG Concentration (mIU/mL)	Expected Doubling Time	Growth Rate (2-Day)
< 1,200	48 − 72 Hours	Increase > 60&percent;
1,200 − 6,000	72 − 96 Hours	Increase > 50&percent;
> 6,000	> 96 Hours	Increase > 35&percent;
> 100,000	Plateau / Slow Decline	Variable

Frequently Asked Questions

Not always. As hCG levels get higher (specifically above 6,000 mIU/mL), the doubling time naturally slows down to 96 hours or more. A common source of anxiety is expecting a 48-hour doubling time when the absolute hCG level is already high.

In early pregnancy, the difference between 40 hours and 48 hours is significant. If your first test was at 8:00 AM Monday and the second was at 4:00 PM Wednesday, that is 56 hours, not 48. Calculating strictly by date ignores this 8-hour gap, leading to inaccurate medical assumptions.

Historically, doctors looked for a 66% increase over 48 hours. Recent studies suggest a more conservative lower limit of 35% rise over 48 hours may still result in a viable pregnancy, though it warrants closer monitoring.

Decreasing hCG levels confirm that the pregnancy is non-viable. The calculator will provide a "Halving Time," which estimates how long it will take for your levels to return to zero (non-pregnant range).