Calculate the percentage change and doubling time between two serum hCG results, and see where the change sits in the NICE criteria for pregnancy of unknown location.1
A summary of NICE guideline NG126 for clinicians, shown for reference. It is not a diagnosis. Results are interpreted by the clinical team alongside symptoms and scan findings, and symptoms take priority over hCG results.1
| Change over 48 hours | What NICE guidance says |
|---|---|
| Increase of more than 63% | A developing pregnancy in the womb is likely, but an ectopic pregnancy cannot be excluded. A transvaginal scan to find the location is offered 7 to 14 days later, or earlier if hCG is 1,500 IU/L or more. Rec. 1.4.29 |
| Decrease of more than 50% | The pregnancy is unlikely to continue, though this is not confirmed. Support information is given, and a urine pregnancy test is taken 14 days after the second blood test. A positive result needs review within 24 hours. Rec. 1.4.30 |
| Anything in between | Clinical review in the early pregnancy assessment service within 24 hours. Rec. 1.4.31 |
hCG results cannot show where a pregnancy is.1 Always follow the advice of your own clinic or early pregnancy unit.
Change = (second − first) ÷ first × 100%
Doubling time = hours between tests × ln 2 ÷ ln(second ÷ first)
NICE recommends two serum hCG measurements as near as possible to 48 hours apart, but no earlier.1 This tool highlights the matching NICE group only for tests taken 48 to 52 hours apart. For other intervals it shows a 48-hour equivalent for information only, which assumes a steady rate of change. It does not diagnose, and it supports rather than replaces clinical judgement.
A single hCG level varies widely between healthy pregnancies, so the change between two tests is more useful. NICE recommends taking the two tests as close as possible to 48 hours apart, and no earlier.1
No. hCG results cannot show where a pregnancy is. Even with a rise of more than 63%, an ectopic pregnancy cannot be ruled out, which is why a scan is offered to check the location.1
NICE groups the change over 48 hours into a rise of more than 63%, a fall of more than 50%, or anything in between. Each group has a different recommended next step, summarised in the table above.1
Yes. NICE advises clinicians to place more importance on symptoms than on hCG results.1 If you feel unwell, seek help whatever your results show.
It is how long the hCG level would take to double if it kept rising at the same rate between your two tests. It is another way of expressing the same change, and does not add information beyond the percentage rise.
The NICE criteria are written for pregnancy of unknown location, when a pregnancy test is positive but no pregnancy is seen on a scan.1 Fertility clinics may check hCG after embryo transfer using their own protocols, so follow the advice your clinic gives you.
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