What is beta-hCG?
Beta-hCG (human chorionic gonadotrophin) is the pregnancy hormone produced by the developing placental tissue after the embryo implants in the lining of the uterus. Blood results are reported in IU/L; some laboratories write the same unit as mIU/mL. The level rises quickly in early pregnancy, peaks at about 8-11 weeks and then falls to a lower level for the rest of the pregnancy.
Why is it measured twice?
Beta-hCG levels vary widely between women at the same stage of pregnancy, so a single value cannot show on its own how many weeks pregnant you are or how the pregnancy is progressing. When the pregnancy cannot yet be seen on ultrasound, when there is bleeding or pain, after IVF, or when ectopic pregnancy or miscarriage is suspected, doctors usually repeat the test about 48 hours later and look at the change between the two results.
How is it calculated?
In early pregnancy beta-hCG is assumed to rise roughly exponentially. From your two results and the time between them, the tool calculates:
- Doubling time (hours) = time × ln(2) ÷ ln(second result ÷ first result)
- Change over 48 hours (%) = [(second result ÷ first result)48 ÷ time − 1] × 100
If the level is falling, the halving time (the time taken for the level to halve) is shown instead of the doubling time. If the tests were not exactly 48 hours apart, the result is adjusted to 48 hours so that it can be compared.
How does beta-hCG usually rise in early pregnancy?
It used to be said that beta-hCG "doubles every two days" in early pregnancy. More recent studies have shown that in healthy pregnancies inside the uterus the rise can be slower than that. The American College of Obstetricians and Gynecologists (ACOG) notes that the slowest rise reported for pregnancies inside the uterus can be about 35% in 48 hours, and this tool uses that cautious threshold in its messages.
- The rate of rise depends on the starting level: the higher the level, the slower the rise and the longer the doubling time. Above a few thousand IU/L the rate of rise gives less information.
- Once beta-hCG reaches roughly 1,500-3,500 IU/L or more, a pregnancy sac can often be seen on a transvaginal scan. From then on, the location and development of the pregnancy are assessed mainly with ultrasound.
- In twin pregnancies levels may be higher on average, but they overlap widely with single pregnancies, so twins cannot be identified from beta-hCG.
How should the result be interpreted?
A rise at the expected rate does not show that the pregnancy is inside the uterus or that it will progress without problems. A slow rise does not always mean there is a problem; it can also occur in some pregnancies that go on normally. However, a slow rise, a level that stays the same or a falling level should be assessed with ultrasound and, if needed, further tests for miscarriage and ectopic pregnancy. After a miscarriage or during treatment of an ectopic pregnancy, the fall in beta-hCG may be followed by your doctor until the test is negative.
The patterns of beta-hCG in pregnancies inside the uterus, ectopic pregnancies and pregnancies ending in miscarriage can overlap. No rate of rise or fall is therefore a diagnosis on its own; the result is assessed together with symptoms, examination and ultrasound findings. For more on interpreting beta-hCG by week and doubling time, see our article on beta-hCG levels, and for follow-up after IVF, beta-hCG after embryo transfer.
Limitations
- Laboratory differences: laboratories may use different assays. Ideally, the two tests being compared are done in the same laboratory.
- Short intervals: with intervals shorter than 24 hours, small measurement differences affect the result more.
- IVF treatment: if the trigger injection contained hCG, part of the level measured in the first days after it may come from the medicine. The clinic running your treatment sets the test date.
- Not a diagnosis: the tool does not show where the pregnancy is, whether it is healthy or how many weeks it is.
When to see a doctor
Always share your beta-hCG results with the doctor who requested them. If you have severe or one-sided abdominal pain, shoulder-tip pain, dizziness, feel faint or have heavy bleeding, go to an emergency department without waiting for results; these can be signs of an ectopic pregnancy or internal bleeding.
Sources
- American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 193: Tubal Ectopic Pregnancy. Obstet Gynecol. 2018;131:e91-e103.
- Barnhart KT, Sammel MD, Rinaudo PF, Zhou L, Hummel AC, Guo W. Symptomatic patients with an early viable intrauterine pregnancy: HCG curves redefined. Obstet Gynecol. 2004;104:50-55.
- Morse CB, Sammel MD, Shaunik A, et al. Performance of human chorionic gonadotropin curves in women at risk for ectopic pregnancy: exceptions to the rules. Fertil Steril. 2012;97:101-106.
- Barnhart KT, Guo W, Cary MS, et al. Differences in serum human chorionic gonadotropin rise in early pregnancy by race and value at presentation. Obstet Gynecol. 2016;128:504-511.
- National Institute for Health and Care Excellence. Ectopic pregnancy and miscarriage: diagnosis and initial management (NG126), 2019, updated 2023.