hCG doubling time calculator

How doubling time is worked out

Early hCG rises roughly exponentially, so two results and the gap between them are enough to solve for how long a doubling would take at that rate:

doubling time = hours between draws x ln(2) / ln(second / first)

A rise from 100 to 200 across 48 hours is a 48-hour doubling time, by definition. A rise from 100 to 153 across the same window is a doubling time of about 78 hours. Those two framings describe the same blood results, which is the reason this page shows both: clinics quote percentage rise over two days, the internet quotes doubling times, and people end up comparing numbers that were never the same measurement.

What the published ranges say

The rule most people arrive with is that hCG doubles every 48 to 72 hours. The research behind the modern reference curves is slower than that at the bottom end, and it has been revised downwards twice.

• Barnhart et al., 2004minimum rise of 53% over two days
• Morse et al., 2012revised down to 35% over two days
• ACOG Practice Bulletin 193discriminatory level set as high as 3,500 mIU/mL

Both of those floors describe the slowest sliver of pregnancies that went on to be viable, not a line between good and bad. Morse and colleagues argued for the more conservative 35% precisely so that a slow-rising pregnancy would not be acted on as though it had already failed. They also found that levels below 500 mIU/mL were an exception to the pattern, which is exactly the range most first betas fall in.

Why one number tells you very little

Individual beta values at any given point after transfer span an enormous range, and a first result on its own predicts far less than people expect it to. The change between two draws carries more information than either draw alone, which is why the second one gets ordered at all. Even then, a case series in F&S Reports documented viable intrauterine pregnancies after embryo transfer whose rate of rise was frankly abnormal. Two numbers are a data point. They are not an outcome.

When bloodwork stops being the test

The rise decelerates as levels climb, so doubling times lengthen on their own and a rate measured at 3,000 is not comparable to one measured at 150. Once levels are high enough for a scan to see a gestational sac, the ultrasound answers questions the bloodwork cannot, and ACOG advises that any discriminatory hCG threshold used for that decision should be set conservatively high to avoid misreading a healthy early pregnancy. Past that point, more betas mostly add anxiety.

The trigger shot

An hCG trigger such as Ovidrel or Pregnyl is hCG, so a test taken too soon after it measures the injection rather than the pregnancy. It clears over roughly one to two weeks depending on the dose, and clinics time the first beta to fall past it. If you are comparing home tests taken in that window, the numbers may be describing the trigger on its way out rather than anything rising.

Sources

• Barnhart KT, et al. Symptomatic patients with an early viable intrauterine pregnancy: hCG curves redefined. Obstet Gynecol. 2004;104(1):50-55.
• Morse CB, et al. Performance of human chorionic gonadotropin curves in women at risk for ectopic pregnancy: exceptions to the rules. Fertil Steril. 2012;97(1):101-106.
• Barnhart KT. What serial hCG can tell you, and cannot tell you, about an early pregnancy. Fertil Steril. 2012.
• Abnormal rate of human chorionic gonadotropin rise: a case series of patients with viable intrauterine pregnancies after embryo transfer. F&S Reports. 2020.
• ACOG Practice Bulletin No. 193: Tubal Ectopic Pregnancy. Obstet Gynecol. 2018.

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