Is my βhCG rising normally?
Enter your two blood test values and the time each was drawn.
Formula: ΔT × ln2 / ln(v2/v1). Reference 48–72 hours applies before about 6 weeks. Learn more
For reference only. This is not a tool to diagnose ectopic pregnancy or miscarriage. Discuss results with your obstetrician.
How is it calculated?
Doubling time is the period needed for βhCG to double. Using two measurements v₁ and v₂ taken ΔT hours apart:
Formula: doubling time = ΔT × ln 2 / ln(v₂ / v₁).
In early pregnancy (3-6 weeks) a normal doubling time is 48-72 hours. After 6 weeks it lengthens — by 9-10 weeks it can approach 100 hours (Barnhart et al., 2004).
Limitations of this estimate
Doubling time is a reference signal and cannot diagnose pregnancy viability on its own:
- About 15% of normal pregnancies rise by less than double in 48 hours (Barnhart 2004).
- If the two measurements are taken too close together (< 24 h), the calculation is highly noisy.
- Lab assays differ — trend within the same lab is the standard.
- After week 6 the doubling time naturally lengthens, so the 48-72 h benchmark no longer applies.
- Ectopic pregnancy or miscarriage workup never relies on doubling time alone — it's combined with ultrasound and symptoms.
Interpretation belongs with your OB-GYN.
Sources
Frequently asked
What does a longer-than-normal doubling time mean?
Before week 6 a normal doubling time is 48-72 hours. Significantly longer, or near-flat values, raises concern for ectopic pregnancy, miscarriage, or non-viable pregnancy — an OB-GYN evaluation is needed.
That said, about 15% of normal pregnancies still rise by less than double in 48 hours, so the diagnosis is never made on a single calculation — it's combined with ultrasound and symptoms.
Is a low absolute hCG number abnormal?
The doubling pattern matters more than the absolute number. βhCG ranges from 5 to 200,000 mIU/mL across the trimesters, and even within "4 weeks pregnant" the value can differ 100-fold between people. So the trend over two measurements is the clinical standard, not a single reading.
If your value is low, the test may simply be too early — your OB-GYN decides when to repeat (typically 48-72 hours later).