hCG calculator: doubling time from two results

Human chorionic gonadotropin (hCG) is the pregnancy hormone. Cells around the implanted embryo start making it, so it shows up in blood a few days after implantation. The quantitative blood test is often called beta-hCG and is reported in mIU/mL (milli-international units per milliliter).

In early pregnancy, doctors rarely focus on a single number, because hCG levels in healthy pregnancies span a huge range. The change between two tests says much more, which is why two draws about 48 hours apart are common. This calculator turns that change into numbers that are easy to read.

In the first weeks, hCG rises roughly exponentially, so the calculator uses logarithms:

• Ratio = second level ÷ first level
• Doubling time = hours between tests × ln 2 ÷ ln(ratio)
• 48-hour change = ratio^(48 ÷ hours) − 1, as a percentage

If the second value is lower, you'll see the percentage drop and the half-life instead. Converting to 48 hours helps because blood draws are rarely exactly 48 hours apart: it lets you compare a 46-hour gap with a 52-hour one.

Your first test shows 120 mIU/mL and the second, 48 hours later, shows 260 mIU/mL.

Step	Calculation	Result
Ratio	260 ÷ 120	2.17
Doubling time	48 × 0.693 ÷ ln 2.17	about 43 hours
48-hour change	2.17 − 1	+117%

If the same result had come 60 hours later, the doubling time would be about 54 hours and the rise converted to 48 hours about +86%. That's why the exact time between draws matters.

The often-quoted "hCG doubles every 48 hours" is a simplification. In a 2004 study of women with symptoms whose early pregnancies turned out to be viable, Barnhart and colleagues found a median 48-hour rise of about 124%, and slower rises also occurred in healthy pregnancies. ACOG guidance stresses that no single rate of rise decides on its own how a pregnancy will go.

The pace also changes with the level: rises are faster at low levels and slow down as hCG climbs. hCG peaks around weeks 8 to 11 and then falls. A slower doubling time at 6 weeks can mean something quite different from the same number at 4 weeks.

The calculator can't tell you whether a pregnancy is healthy, where it is located or how many embryos there are. It doesn't label any result "normal" or "abnormal," because that takes your symptoms, an ultrasound and your full history.

Above roughly 1,500–2,000 mIU/mL, a transvaginal ultrasound can usually show a gestational sac, and at that point imaging tells more than hCG numbers. Don't compare results from different labs, since methods vary. The weekly table is only a guide: your lab's own reference range is what counts.

With a positive test or rising hCG, get urgent care if you have:

• severe or one-sided belly pain
• pain at the tip of your shoulder
• dizziness or feeling faint
• heavy bleeding

These can be signs of an ectopic pregnancy, which needs emergency treatment. Call 911 or go to the nearest emergency room.