hCG doubling time calculator: how fast are my levels rising?
Doubling time: 1.8 days (43 hours)
That’s a 117% change over 48 hours. In early pregnancy, hCG usually rises by at least 35 to 50% every 48 hours, though a slower rise can still be normal. Only your doctor can interpret your results alongside a scan.
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An hCG doubling time calculator uses two blood results and the hours between them: doubling time = hours × ln(2) ÷ ln(second result ÷ first result). A rise from 120 to 260 mIU/mL in 48 hours is a doubling time of about 43 hours. In early pregnancy, levels usually rise by at least 35 to 50% every 48 hours, but only your clinic can interpret your results.
How the hCG doubling time calculator works
hCG (human chorionic gonadotropin) is the hormone a pregnancy test detects. In early pregnancy, doctors sometimes measure it in the blood twice, usually 48 hours apart, to see how it's changing. The trend matters far more than any single number.
- Enter your first beta hCG result in mIU/mL (the same as IU/L).
- Enter your second result.
- Set the hours between the two blood draws (12 to 168 hours).
- Read the doubling time in hours and days, and the percentage change scaled to 48 hours. If the level fell or stayed flat, it says so instead.
The formulas
Doubling time = hours between tests × ln(2) ÷ ln(second ÷ first). 48-hour rise (%) = ((second ÷ first) ^ (48 ÷ hours) − 1) × 100. The second formula lets you compare results taken 36 or 72 hours apart with the usual 48-hour guidance.
A worked example
Your first beta is 120 mIU/mL and your second, 48 hours later, is 260 mIU/mL. 260 ÷ 120 = 2.17. ln(2) ÷ ln(2.17) × 48 hours ≈ 43 hours (1.8 days), a rise of about 117% over 48 hours.
| First result | Second (48 h later) | 48-hour rise | Doubling time |
|---|---|---|---|
| 100 | 200 | 100% | 48 hours (2.0 days) |
| 100 | 160 | 60% | about 71 hours (2.9 days) |
| 100 | 135 | 35% | about 111 hours (4.6 days) |
| 100 | 100 | 0% | no rise |
What's a normal hCG rise?
Older teaching said hCG should double every 48 hours. Larger studies found that many healthy pregnancies rise more slowly. In Barnhart and colleagues' 2004 study of people with pain or bleeding who went on to have a healthy pregnancy, the slowest rise was about 53% in 2 days. ACOG uses about 35% in 48 hours as a conservative minimum, and NICE guidance treats a rise of more than 63% in 48 hours as likely to be a developing pregnancy in the uterus, while noting it doesn't rule out an ectopic pregnancy.
- Doubling slows down as levels rise. Above several thousand mIU/mL, a slower rise is expected, and hCG peaks around 8 to 11 weeks before falling.
- A slow rise isn't a diagnosis. It can happen in healthy pregnancies, and it can also be an early sign of miscarriage or ectopic pregnancy. That's why clinics combine hCG with an ultrasound.
- Different labs, small differences. Compare results from the same lab when you can.
This calculator does the maths, nothing more. It can't tell you whether a pregnancy is healthy. Your doctor or fertility clinic reads your numbers together with your symptoms, history and scan.
Beta hCG levels by week
Typical ranges counted from the first day of your last period. The ranges are huge, and a single number says very little: two people with healthy pregnancies at the same week can differ tenfold.
| Weeks since last period | hCG (mIU/mL) |
|---|---|
| 3 weeks | 5 to 50 |
| 4 weeks | 5 to 426 |
| 5 weeks | 18 to 7,340 |
| 6 weeks | 1,080 to 56,500 |
| 7 to 8 weeks | 7,650 to 229,000 |
| 9 to 12 weeks | 25,700 to 288,000 |
| 13 to 16 weeks | 13,300 to 254,000 |
| 17 to 24 weeks | 4,060 to 165,400 |
| 25 to 40 weeks | 3,640 to 117,000 |
Source: Cleveland Clinic. Under 5 mIU/mL usually means not pregnant. If you're not sure how far along you are, the pregnancy calculator gives your weeks from your last period.
Waiting for results
The 48 hours between blood tests can feel endless. It's common to feel anxious, low or unable to focus, especially if you've had a loss before or you're in fertility treatment. Be gentle with yourself, and lean on someone you trust. If you're tracking an IVF pregnancy, the IVF due date calculator and the DPO calculator can help with dates while you wait.
You have one-sided or severe pelvic pain, shoulder-tip pain, heavy bleeding, dizziness, fainting or diarrhea and pain when going to the toilet. These can be signs of an ectopic pregnancy, which needs care straight away whatever your hCG numbers say.
Frequently asked questions
What is a normal hCG doubling time?
In very early pregnancy, often about 48 to 72 hours, slowing as levels climb. Many healthy pregnancies rise more slowly than doubling, so your clinic looks at the trend alongside a scan.
What does it mean if my hCG isn't doubling?
It doesn't automatically mean something is wrong. A slower rise can happen in healthy pregnancies, but it can also be an early sign of miscarriage or ectopic pregnancy, so your clinic will usually repeat the test and arrange a scan.
What hCG level confirms pregnancy?
A blood level above about 25 mIU/mL is generally considered positive, and under 5 is usually negative. Levels in between are often retested.
How long should I wait between hCG tests?
Usually 48 hours, as your clinic advises. This calculator scales any gap to a 48-hour rise so results can be compared.
Sources
- Barnhart KT et al. Symptomatic patients with an early viable intrauterine pregnancy: HCG curves redefined. Obstetrics & Gynecology (2004)
- NICE NG126: Ectopic pregnancy and miscarriage: diagnosis and initial management
- ACOG Practice Bulletin 193: Tubal Ectopic Pregnancy
- Cleveland Clinic: hCG (human chorionic gonadotropin)
This page is for information only and isn’t medical advice. If something worries you, please talk to a doctor.
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