Early pregnancy · hCG
hCG doubling time calculator.
Enter two hCG blood results and the hours between the draws to see the percentage rise, the doubling time, and the rise scaled to 48 hours.
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hCG Doubling Time Calculator: how it works
hCG doubling time is the hours between two blood tests multiplied by ln 2, divided by ln of (second result / first result). For example, 600 rising to 1,500 mIU/mL in 48 hours is a 150% rise and a doubling time of 36.3 hours. The calculator also scales the rise to 48 hours so it can be set beside published reference rises.
The doubling time assumes steady exponential growth between the two draws: doubling time = hours x ln 2 / ln(second / first). The 48 hour rise is (second / first) raised to the power 48 / hours, minus one. Both are arithmetic summaries of two numbers, not a prediction.
The reference rises come from Barnhart and colleagues (Obstetrics and Gynecology, 2016), who studied 285 women with first-trimester pain and bleeding and a pregnancy of unknown location who were later confirmed to have a normal intrauterine pregnancy. For starting hCG values of under 1,500, 1,500 to 3,000 and over 3,000 mIU/mL, the predicted 2-day minimal (first percentile) rise was 49%, 40% and 33%. These are ranges seen in a particular group of symptomatic patients. They are not a pass/fail line for everyone, and a slower rise does not on its own show a problem. Different labs report slightly different values, so compare results from the same lab.
Worked example. First result 600 mIU/mL, second result 1,500 mIU/mL, 48 hours apart. The ratio is 1,500 / 600 = 2.5, a 150% rise. Doubling time = 48 x ln 2 / ln 2.5 = 36.3 hours. The first value is under 1,500, so the reference rise is 49% in 2 days, and 150% is above it. A second case: 2,000 rising to 2,600 in 48 hours is a 30% rise, below the 40% reference for that starting range, which is a prompt to talk to your clinician and not a diagnosis.
Frequently asked questions
How do you calculate hCG doubling time?
Doubling time = hours between draws x ln 2 / ln(second result / first result). For 600 to 1,500 in 48 hours that is 36.3 hours.
What is a normal hCG rise?
There is no single normal number. In one study of women with viable pregnancies, the slowest 1% rose by at least 49%, 40% or 33% in 2 days depending on a starting value under 1,500, between 1,500 and 3,000, or over 3,000 mIU/mL. Many rose faster.
Does a slow rise mean a miscarriage or ectopic pregnancy?
Not by itself. A slower rise can have several explanations and some viable pregnancies rise slowly. Only your clinician can interpret it, usually with repeat tests and an ultrasound.
Why does the calculator show the rise over 48 hours?
Published reference rises are quoted per 2 days, so the calculator scales your own interval to 48 hours assuming a steady rate. The comparison is only shown for draws about 24 to 96 hours apart.
When should I call my clinician?
Call promptly for pelvic or abdominal pain, heavy bleeding, shoulder-tip pain, or dizziness and fainting in early pregnancy. Do not wait for another blood test.
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Not medical advice
Not medical advice. This tool does not diagnose anything and cannot tell you whether a pregnancy is healthy or whether a cycle is normal for you. See your clinician for heavy or unusual bleeding, pain, dizziness, or any symptom that worries you, and get urgent care for severe pain or very heavy bleeding.