Unit Converter
Free β-subunit of human chorionic gonadotropin (free βhCG)
(Placenta-Derived Glycoprotein – Key Marker in Down Syndrome Screening, Pregnancy Disorders & Tumors)
Synonyms
- Free β-hCG
- β-hCG (free beta subunit)
- Uncombined β-subunit
- Free beta human chorionic gonadotropin
- β-core hCG fragment (related)
- Placental hCG marker
Units of Measurement
- ng/mL
- mIU/mL
- IU/L
Unit Conversions
1 mIU/mL = 1 IU/L
1 ng/mL (free β-hCG) ≠ directly convertible to IU units
Because IU measures biological activity, and ng measures mass.
Conversion varies by assay and manufacturer.
Important:
Clinical labs do not convert between ng/mL ↔ mIU/mL for free β-hCG.
Each unit must be interpreted as reported by the assay.
Description
Free β-hCG is the uncombined beta subunit of the hormone hCG.
It is produced predominantly by:
- Syncytiotrophoblasts of the placenta
- Trophoblastic tumors
- Certain germ cell tumors
Free β-hCG is biologically active and is a more specific marker than total hCG in some conditions.
It is widely used for:
- First-trimester aneuploidy screening (Down syndrome risk assessment)
- Early pregnancy evaluation
- Diagnosis & monitoring of trophoblastic disease
- Tumor marker in germ cell tumors
Physiological Role
Although total hCG supports:
- Corpus luteum maintenance
- Progesterone production
- Embryo implantation
The free β-subunit has no independent physiologic function, but elevated levels are clinically meaningful.
Clinical Significance
High Free β-hCG
1. Down Syndrome (Trisomy 21)
One of the most important applications.
In the first-trimester combined screen (11–14 weeks):
- Free β-hCG is elevated
- NT (nuchal translucency) ↑
- PAPP-A ↓
Used to calculate MoM (Multiple of Median):
- Free β-hCG MoM > 2.0 → ↑ risk for Trisomy 21
2. Trophoblastic Diseases
- Complete molar pregnancy
- Partial molar pregnancy
- Persistent gestational trophoblastic disease
- Choriocarcinoma
Very high free β-hCG values are typical.
3. Germ Cell Tumors
- Testicular tumors (non-seminomatous)
- Ovarian germ cell tumors
- Mediastinal germ cell tumors
Free β-hCG serves as a tumor marker.
4. Normal Pregnancy
Increases rapidly:
- Peaks around 10–12 weeks
- Then declines and stabilizes
Free β-hCG values are higher than in non-pregnant states.
Low Free β-hCG
Associated with:
- Trisomy 18 (Edwards syndrome)
- Trisomy 13 (Patau syndrome)
- Potential miscarriage
- Non-viable pregnancy
- Ectopic pregnancy (may be lower than expected)
Reference Intervals
(Assay- and gestational-age dependent; MoM values are standard.)
In Pregnancy (First Trimester: 11–14 weeks)
Results reported as:
- MoM (Multiple of Median)
- ng/mL or IU/L depending on assay
Typical MoM Ranges
| Condition | Free β-hCG MoM |
| Normal pregnancy | 0.5 – 2.0 MoM |
| Down Syndrome (T21) | >2.0 MoM |
| Trisomy 18/13 | <0.5 MoM |
Absolute numeric reference intervals (ng/mL or IU/L) are not clinically standardized due to wide assay variability.
Non-Pregnant Adults
- Free β-hCG: Undetectable or <1–2 IU/L
Postmenopausal Women
- May have low detectable levels (<7 IU/L) due to pituitary hCG.
Interpretation Note
Because of assay variability, use:
- MoM values
- Gestational age-specific medians
These are standard in prenatal screening.
Diagnostic Uses
1. First-Trimester Aneuploidy Screening
Combined with:
- Nuchal translucency (NT)
- PAPP-A
- Maternal age
Free β-hCG is a critical marker for Down syndrome screening.
2. Diagnosis of Trophoblastic Disease
- Extremely elevated levels
- Used to monitor treatment response
- Detect recurrence of GTD
3. Tumor Marker
For:
- Testicular cancer
- Ovarian germ cell tumors
- Mediastinal germ cell tumors
Trend monitoring is essential.
4. Early Pregnancy Assessment
- Helps identify abnormal rise in failing pregnancy
- Lower levels → ectopic or early loss (not diagnostic alone)
Analytical Notes
- Serum preferred
- Use same assay for serial measurements
- Free β-hCG is more specific for Down syndrome than total hCG
- Biotin supplements can interfere with immunoassays
Clinical Pearls
- Free β-hCG is elevated in Down syndrome, while PAPP-A is decreased.
- hCG-related tumors produce disproportionately high free β-hCG.
- Always interpret pregnancy results using MoM, not raw units.
- Pituitary hCG can cause low-level false positives in postmenopausal women.
- Very high free β-hCG in early pregnancy may indicate molar pregnancy.
Interesting Fact
The beta subunit of hCG contains the immunologically unique region that distinguishes hCG from LH - allowing highly specific assays for early pregnancy and tumor detection.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - hCG Variants.
- ACOG & SMFM Guidelines - First Trimester Screening.
- ISPD (International Society for Prenatal Diagnosis) Standards.
- Mayo Clinic Laboratories - Free β-hCG.
- ARUP Consult - hCG Interpretation.
- MedlinePlus / NIH - hCG Test.