What hCG doubling time tells you
Human chorionic gonadotropin (hCG) is the hormone produced after implantation. In early pregnancy, levels rise quickly, and doctors sometimes order two blood tests 48 hours apart to see how fast they increase. This calculator turns two results into a doubling time and a percentage rise over 48 hours.
The formulas
- Doubling time = hours between the tests × ln 2 ÷ ln(second result ÷ first result)
- Rise over 48 hours = (second ÷ first)^(48 ÷ hours) − 1
The 48-hour rise lets you compare tests taken at different intervals.
Worked example
The first test shows 120 mIU/mL and the second, 48 hours later, 250 mIU/mL.
- Ratio: 250 ÷ 120 = 2.08.
- Doubling time: 48 × 0.693 ÷ ln(2.08) = 45 hours, or 1.9 days.
- Rise over 48 hours: 108%.
What is a normal rise?
In a healthy early pregnancy, hCG often doubles every 48 to 72 hours. A large study found that most viable pregnancies rise by at least 35% in 48 hours when starting from low levels; slower rises are more often seen with an ectopic pregnancy or a pregnancy that will not continue, but some healthy pregnancies rise slowly too.
| Starting level (mIU/mL) | Usual minimum 48-hour rise |
|---|---|
| under 1,500 | about 49% |
| 1,500 to 3,000 | about 40% |
| over 3,000 | about 33% |
These figures from published research show that the expected rise slows as levels climb.
hCG during pregnancy
| Weeks since the last period | Typical range (mIU/mL) |
|---|---|
| 3 | 5 to 50 |
| 4 | 5 to 426 |
| 5 | 18 to 7,340 |
| 6 | 1,080 to 56,500 |
| 7 to 8 | 7,650 to 229,000 |
Ranges are very wide, so a single value says little: the trend matters more. Levels peak around 8 to 11 weeks and then fall.
Limits
Doubling time is useful mainly in the first weeks. Once hCG passes about 1,500 to 3,500 mIU/mL, a transvaginal ultrasound should show a gestational sac in the uterus and becomes the main test. Always discuss results with your provider, especially with pain or bleeding.
A note on medical advice
This calculator gives estimates based on averages. It does not replace advice from your doctor, midwife or health care provider, who will date your pregnancy and follow it with exams and ultrasounds.