hCG Unit Converter
hCG Unit Converter
mIU/mL, IU/L and mU/mL are the same number for hCG — the useful part of this page is the discriminatory zone, the doubling rule and the hook effect.
hCG converter
IU/L = mIU/mLhCG 1,500 mIU/mL
Units and what they mean
mU/mL = mIU/mL × 1
All three units are numerically identical
- × 1
- one milli-international unit per millilitre is one international unit per litre; the conversion is a relabelling and the number never changes
- no molar unit
- hCG is a heterodimer with variable glycoforms, standardised in international units against a WHO reference preparation, so no molar conversion is meaningful
- discriminatory zone
- the level above which an intrauterine pregnancy should be visible on transvaginal ultrasound; now recommended to be set high, around 3,500 mIU/mL
- hook effect
- very high concentrations saturate both antibodies in a sandwich assay and produce a falsely low result; diluting the sample reveals the true value
Worked example
hCG 1,500 mIU/mL
1,500 mIU/mL = 1,500 IU/L — the units are identical, so the figure is unchanged
= 1,500 mU/mL
Above the non-pregnant interval of 0–5 mIU/mL; in a pregnancy of unknown location this value alone does not exclude a viable intrauterine pregnancy
Typical hCG by gestational age (identical in mIU/mL, IU/L and mU/mL)
| Weeks from last menstrual period | hCG |
|---|---|
| 3 | 5 – 50 |
| 4 | 5 – 426 |
| 5 | 18 – 7,340 |
| 6 | 1,080 – 56,500 |
| 7 – 8 | 7,650 – 229,000 |
| 9 – 12 | 25,700 – 288,000 |
| 13 – 16 | 13,300 – 254,000 |
| 17 – 24 | 4,060 – 165,400 |
| 25 – 40 | 3,640 – 117,000 |
Three traps in reading an hCG
| Trap | What happens | What to do |
|---|---|---|
| Low discriminatory zone | An intrauterine pregnancy is assumed absent at 1,500 – 2,000 mIU/mL and a viable pregnancy is interrupted | Use a higher threshold, around 3,500 mIU/mL, and repeat imaging rather than acting on one value |
| Doubling rule applied too high | A normal pregnancy above about 6,000 mIU/mL fails to double in 48 hours and is wrongly called non-viable | Apply the rule only at lower levels, and note that even below that the minimum normal rise is well under a doubling |
| Hook effect | Very high hCG saturates both antibodies of the sandwich assay and the reported value is falsely low or negative | Request a diluted repeat when the clinical picture — molar pregnancy, choriocarcinoma — does not fit the number |
No conversion, three traps
There is nothing to convert. hCG is reported in mIU/mL, IU/L and mU/mL, and all three are numerically identical, because one milli-international unit per millilitre is one international unit per litre. An hCG of 1,500 mIU/mL is 1,500 IU/L. No molar unit is offered: hCG is a heterodimeric glycoprotein with variable glycoforms, standardised by bioactivity in international units against a WHO reference preparation, so a molar concentration would have no single meaningful value. The value of this page lies in the three ways the number is misread.
The first is the discriminatory zone, the level above which an intrauterine pregnancy should be visible on transvaginal ultrasound. It is commonly quoted as 1,500 to 2,000 mIU/mL, but current guidance is that it should be applied cautiously and set considerably higher, around 3,500, when the question is whether a pregnancy of unknown location can be treated as non-viable. Viable intrauterine pregnancies, particularly multiples, have been interrupted by acting on a low threshold, and no single value should be used to justify intervention when the patient is stable and can be re-imaged.
The second is the doubling rule. hCG roughly doubles every 48 hours in early normal pregnancy, but that holds only below about 6,000 mIU/mL, after which the rise slows markedly; applying it at higher levels will label normal pregnancies as failing. Even in the early range, the minimum rise consistent with a viable pregnancy is considerably less than a doubling, so a slower rise is not by itself evidence of ectopic or non-viable pregnancy.
The third is analytical and the most dangerous. At very high concentrations both the capture and the detection antibody of a sandwich immunoassay become saturated, the sandwich fails to form, and the reported result is falsely low or even negative. This hook effect occurs in exactly the settings where the true value is enormous: molar pregnancy and choriocarcinoma. When the clinical picture does not fit a modest hCG, ask the laboratory for a diluted repeat.
Frequently asked questions
Is mIU/mL the same as IU/L for hCG?
Yes, exactly. One milli-international unit per millilitre equals one international unit per litre, and mU/mL is the same again. An hCG of 1,500 mIU/mL is 1,500 IU/L, with no calculation required.
Why is hCG not reported in pmol/L?
hCG is a heterodimeric glycoprotein with variable glycoforms and no single meaningful molecular mass for reporting. It is standardised by bioactivity in international units against a WHO reference preparation, so molar units are not used.
What is the discriminatory zone and which value should be used?
It is the hCG above which an intrauterine pregnancy should be visible on transvaginal ultrasound. It is often quoted as 1,500 to 2,000 mIU/mL, but current guidance recommends using a higher value, around 3,500, before treating a pregnancy of unknown location as non-viable, because a low threshold risks intervening in a viable pregnancy.
Does hCG always double every 48 hours?
No. The approximate doubling applies in early pregnancy below about 6,000 mIU/mL, and the rise slows markedly above that. Even in the early range the minimum rise consistent with viability is less than a doubling, so a slower rise alone does not diagnose a failing or ectopic pregnancy.
What is the hook effect?
At very high concentrations hCG saturates both antibodies of a sandwich immunoassay, so the sandwich cannot form and the reported result is falsely low or negative. It matters in molar pregnancy and choriocarcinoma, and is resolved by repeating the assay on a diluted sample.
Related calculators
References
- American College of Obstetricians and Gynecologists. Practice Bulletin No. 193: tubal ectopic pregnancy. Obstet Gynecol. 2018;131(3):e91–e103.
- National Institute for Health and Care Excellence. Ectopic pregnancy and miscarriage: diagnosis and initial management. NICE guideline NG126. 2019, updated 2023.
- Doubilet PM, Benson CB, Bourne T, et al. Diagnostic criteria for nonviable pregnancy early in the first trimester. N Engl J Med. 2013;369(15):1443–1451.