hCG doubling time calculator
Enter two hCG results with the date and time of each blood draw. The tool works out your doubling time in hours, the percentage rise, and how that compares with the ranges actually recorded for early pregnancy. It cannot tell you where a pregnancy is or whether it will continue — only a scan and repeat bloods can do that.
hCG doubling time calculator
No sign-up · PrivateThe number on your report. mIU/mL and IU/L are the same unit — either is fine, as long as both results come from the same laboratory.
Doubling time is measured in hours, so the clock times change the answer. If you cannot remember, leave both at 09:00 — the error mostly cancels out.
Counted from the first day of your last period. Optional — it only changes the interpretation, never the arithmetic.
Enter both results and the date of each blood draw. Everything is worked out in your browser — no result is sent anywhere.
Nothing you type leaves your device. The whole calculation runs in your browser.
How this is calculated
FormulahCG rises exponentially in early pregnancy, so doubling time is not a subtraction — it comes from the exponential relation between the two results and the hours between them:
doubling time (hours) = (hours between draws × ln2) ÷ ln(second result ÷ first result)
where ln is the natural logarithm and ln2 ≈ 0.693147. Worked example: 240 mIU/mL rising to 520 mIU/mL across 48 hours gives (48 × 0.693147) ÷ ln(2.1667) = 33.271 ÷ 0.77319 = 43.0 hours.
Your two blood draws are almost never exactly 48 hours apart, and every published threshold is written over two days. So before comparing anything, the tool converts your actual interval into its 48-hour equivalent:
48-hour equivalent rise (%) = ((second ÷ first) ^ (48 ÷ hours) − 1) × 100
Worked example: 240 mIU/mL to 400 mIU/mL across 30 hours is a 67% rise over those 30 hours, but 1.6667 ^ 1.6 = 2.2644, so the equivalent 48-hour rise is +126.4%.
When the second result is lower than the first, the same relation is used with the ratio inverted, which yields a half-life instead: half-life (hours) = (hours × ln2) ÷ ln(first ÷ second). 1,800 mIU/mL falling to 700 mIU/mL over 48 hours gives a half-life of 35.2 hours.
That 48-hour figure is then compared against measured thresholds rather than the folk rule. The slowest two-day rise recorded in a confirmed viable intrauterine pregnancy depends on the starting value: 49% below 1,500 mIU/mL, 40% from 1,500 to 3,000, and 33% above 3,000 (Barnhart 2016; the same figures appear in ACOG Practice Bulletin 193). NICE NG126 uses a rise above 63% in 48 hours as consistent with a developing intrauterine pregnancy, a fall greater than 50% as a pregnancy unlikely to continue, and anything between the two as grounds for review in an early pregnancy assessment unit within 24 hours.
Two deliberate refusals. If the draws are less than 12 hours apart, no 48-hour figure and no band verdict is given: every threshold above was derived from samples about two days apart, and projecting from an 8-hour gap produces a large, confident-looking number that means nothing. If the first value is above 6,000 mIU/mL or you are past about 7 weeks, the minimum-rise thresholds are not applied at all, because they were derived from early pregnancies presenting at low levels — hCG slows on its own beyond that point, and grading a normal late rise against an early-pregnancy floor would manufacture alarm out of ordinary physiology.
All dates are entered as separate day, month and year boxes and handled in UTC, so a clock change between two draws cannot add or remove an hour. Nothing is transmitted or stored; the whole calculation runs in your browser.
Between two blood tests there is nothing to do but wait, and the number in front of you feels like the only piece of information in the world. It is worth knowing exactly how much information it actually is.
What a doubling time measures
Human chorionic gonadotrophin (hCG) is produced by the cells that go on to form the placenta, beginning at implantation. In early pregnancy it climbs roughly exponentially, which is why the meaningful measure is not how many units it gained but how long it takes to double. Two results and the clock times between them are enough to work that out.
Because two draws are rarely exactly 48 hours apart, this tool converts your interval into a 48-hour equivalent before comparing it to anything. Every published threshold is written over two days, so holding a 30-hour gap up against them directly would be comparing different things.
The 48-hour doubling rule is folklore
The belief that hCG "must double every 48 hours" descends from small studies conducted around 1981, when serial hCG was new and the cohorts numbered in the dozens. It was never a rule that all healthy pregnancies obeyed. Forty years of larger work has pushed the floor steadily downwards — one landmark 2004 paper is titled, plainly, HCG curves redefined.
What the minimum rise actually is
The current reference figures come from a study of 285 women who arrived with pain or bleeding, had an inconclusive scan, and were subsequently confirmed to have a normal pregnancy in the womb. The slowest rise observed — the first percentile — depended on where the level started:
- Below 1,500 mIU/mL — at least 49% over two days
- 1,500 to 3,000 mIU/mL — at least 40%
- Above 3,000 mIU/mL — at least 33%
ACOG’s practice bulletin on tubal ectopic pregnancy carries these same figures. A separate analysis of 1,005 pregnancies of unknown location settled on a 35% two-day rise as the best single cut-off. Nobody working from evidence uses 100% any more.
These are floors, not targets. A 49% rise is not "good" — it is the slowest rise recorded in that cohort among pregnancies that turned out perfectly fine. Most normal pregnancies rise a great deal faster. Sitting just above the floor is not the same as being reassured, and sitting just below it is not a diagnosis.
What the UK guideline does with your number
NICE guideline NG126 gives clinicians three bands for two results taken 48 hours apart, when a pregnancy has not yet been located on a scan:
- Rise greater than 63% — likely a developing pregnancy in the womb, although an ectopic pregnancy cannot be excluded. A scan is offered 7 to 14 days later, or earlier once hCG reaches 1,500 IU/L.
- Fall greater than 50% — the pregnancy is unlikely to continue, though this is not confirmed. Follow-up continues until hCG returns to zero.
- Anything in between — review in an early pregnancy assessment unit within 24 hours.
NICE is also explicit about something worth repeating: in a pregnancy of unknown location, clinical symptoms carry more weight than hCG results. If you feel unwell, the number does not overrule that.
When the level is falling
A falling hCG has its own arithmetic — a half-life rather than a doubling time. Falls are steeper from higher starting levels; the reference work puts the expected two-day drop somewhere around 36% to 47% depending on where it began. A slower fall than that is not the good news it might sound like, because a miscarriage resolving incompletely and an ectopic pregnancy can both produce a sluggish decline. It is the reason follow-up continues to zero rather than stopping once the number is simply "going the right way".
Why the rise slows later on
hCG does not climb at one steady rate throughout. Doubling time lengthens as the concentration rises and as the weeks pass. Beyond roughly 6,000 mIU/mL, and after about 6 to 7 weeks, three or four days between doublings is unremarkable. hCG peaks somewhere around 8 to 11 weeks and then declines for the rest of the pregnancy, which is entirely normal and not a sign of anything going wrong. At that stage serial bloods stop being the right test and a scan becomes it. This tool declines to grade results in that range against thresholds that were never derived for them.
What two numbers cannot tell you
In that 1,005-pregnancy analysis, serial hCG used on its own misclassified 16.8% of ectopic pregnancies and 7.7% of pregnancies that were entirely normal. Adding a third result and an early scan brought the second figure down to 2.7%. A doubling time narrows the possibilities. It does not close them, and it cannot locate a pregnancy.
Two practical points. Compare results from the same laboratory only — hCG assays differ between manufacturers, and switching labs mid-series can produce an apparent jump or drop that never happened. And absolute values matter far less than the trend: the normal range at any given week spans orders of magnitude between individuals, which is why a number that looks low next to a chart online may be entirely unremarkable for you.
Get seen, whatever the number says
Sudden or severe pain on one side of your tummy, pain at the tip of your shoulder, feeling faint or collapsing, or heavy bleeding needs emergency care straight away — call 999 or go to A&E. Do not wait for the next blood test.
And if you are waiting, or if this has not gone the way you hoped: Tommy’s midwives (0800 0147 800), Miscarriage UK, formerly the Miscarriage Association (0303 003 6464), and The Ectopic Pregnancy Trust (0300 102 0180) all run free lines staffed by people who have this conversation every day. There are no ads on this page and there never will be.
Sources
- Differences in Serum Human Chorionic Gonadotropin Rise in Early Pregnancy by Race and Value at Presentation (Barnhart KT et al., Obstet Gynecol 2016;128(3):504-511) — Obstetrics & Gynecology / PubMed, accessed
- Symptomatic patients with an early viable intrauterine pregnancy: HCG curves redefined (Barnhart KT et al., Obstet Gynecol 2004;104(1):50-55) — Obstetrics & Gynecology / PubMed, accessed
- Performance of human chorionic gonadotropin curves in women at risk for ectopic pregnancy: exceptions to the rules (Morse CB et al., Fertil Steril 2012;97(1):101-106) — Fertility and Sterility / PubMed, accessed
- Ectopic pregnancy and miscarriage: diagnosis and initial management (NG126) — Diagnosis of viable intrauterine pregnancy and of tubal ectopic pregnancy — NICE, accessed
- Tubal Ectopic Pregnancy (Practice Bulletin 193) — ACOG, accessed
- Early Pregnancy Loss (Practice Bulletin 200) — ACOG, accessed
- Ectopic pregnancy — Diagnosis — NHS, accessed
- β-Human Chorionic Gonadotropin Dynamics in Early Gestational Events: A Practical and Updated Reappraisal (Larraín D, Caradeux J, Obstet Gynecol Int 2024) — Obstetrics and Gynecology International / PubMed, accessed
- Baby loss information and support — Tommy’s, accessed
- Miscarriage UK (The Miscarriage Association) — Miscarriage UK, accessed
- The Ectopic Pregnancy Trust — The Ectopic Pregnancy Trust, accessed