hCG (human chorionic gonadotropin)
Also known as: beta-hCG, human chorionic gonadotropin, pregnancy hormone, serum hCG
The hormone made from implantation onwards by the trophoblast, the tissue that becomes the placenta. It is what pregnancy tests detect, and two blood levels about 48 hours apart, read with an ultrasound scan, show how an early pregnancy is behaving. Outside pregnancy it is a tumour marker.
- Also known as
- beta-hCG, human chorionic gonadotropin, pregnancy hormone, serum hCG
- Units
- IU/L in the UK (international units per litre), reported as mIU/mL in the United States, where 1 mIU/mL is the same as 1 IU/L; urine tests are reported as positive or negative rather than as a number
- Measured by
- Pregnancy test
What it is and what it does
Human chorionic gonadotropin is a glycoprotein whose beta subunit is unique to it, which is why laboratory tests measure beta-hCG. It is secreted by the syncytiotrophoblast of the implanting embryo and later by the placenta, and keeps the corpus luteum making progesterone so that the endometrium is not shed, until the placenta takes over at about 10 to 12 weeks. hCG appears in blood within a few days of implantation and in urine at about the time of the missed period, when a home test becomes reliable. In a normally developing early pregnancy the level roughly doubles every two to three days, so what matters is the trend between two samples rather than any single value, read alongside an ultrasound scan and the person's symptoms. Sudden severe one-sided pelvic pain, shoulder-tip pain, faintness or collapse in early pregnancy needs emergency assessment and should not wait for a repeat blood test.
Why it may be higher
- Pregnancy: the level rises steeply with gestational age in the first weeks and is the basis of every pregnancy test
- A multiple pregnancy, in which the level is higher than expected for the dates
- A molar pregnancy and other gestational trophoblastic disease, in which the level is often very high and fails to fall as expected
- Germ cell tumours of the testis or ovary, and rarely other cancers, where hCG serves as a tumour marker
- A recent pregnancy, miscarriage or termination, after which the level takes days to weeks to fall, so a test can stay positive for a time
- Fertility treatment using an injection that contains hCG to trigger ovulation
Why it may be lower
- An ectopic pregnancy, in which the level often rises more slowly than expected or plateaus instead of doubling; a normal rise does not rule an ectopic pregnancy out, and hCG cannot show where a pregnancy is, so pain or bleeding in early pregnancy needs assessment in an early pregnancy unit whatever the number does
- Miscarriage, in which the level falls
- A pregnancy earlier than the dates suggest, or a test done too soon after conception, when the level is still below what the test can detect
- The second and third trimesters of a normal pregnancy, when the level falls from its first-trimester peak to a lower plateau
- A falling level after treatment of a molar pregnancy or a germ cell tumour, which is the sign that treatment is working
A high or low value can be associated with several different situations. The result is interpreted with other results, symptoms, history and clinical findings.
What can affect the result
- How long it is since implantation: a urine test taken before the missed period can be negative in a perfectly normal pregnancy and should be repeated
- Dilute urine, which is why the first urine of the day is advised when testing early
- Fertility treatment containing hCG, which can keep a test positive for several days after the injection
- Rare interfering antibodies in blood, which can give a persistently positive result with no pregnancy or tumour to explain it
- The assay and the units differ between laboratories, so a series of results should be compared only within the same laboratory