Biomarker 101: Human chorionic gonadotropin (hCG)

hCG is the hormone every pregnancy test detects. A blood test measures the same thing, but as a precise number rather than a yes or no.

What hCG is

Human chorionic gonadotropin is produced by the placenta during pregnancy. Its main role early on is stimulating progesterone production, which maintains the uterine lining.

It can be detected in both urine and blood. Urine tests give a yes-or-no answer; a blood test gives a quantitative figure, which allows levels to be compared over time.

How levels behave

In people who are not pregnant, hCG is very low. In early pregnancy it rises rapidly, roughly doubling every two to three days in the first weeks. It peaks somewhere around nine to ten weeks and then declines gradually through the remainder of the pregnancy.

The ranges at any given week are extremely wide. Two entirely healthy pregnancies at the same stage can produce very different figures, which is why a single hCG value says far less than the pattern across two samples taken a couple of days apart.

Why it's measured

hCG is measured to confirm pregnancy, particularly where an early or precise answer is needed. It is also used to follow a pregnancy's progression when there is a reason for closer monitoring, and it is sometimes measured in other clinical contexts unrelated to pregnancy.

Timing and detection

hCG becomes detectable roughly ten days after conception. Blood tests detect lower concentrations than urine tests, so they can confirm a pregnancy slightly earlier.

Because levels rise at different rates between individuals, testing very early can produce a negative result in a pregnancy that is progressing normally. Testing after a missed period gives a more reliable answer.

What can affect the result

Higher-than-expected levels can occur with a multiple pregnancy, since more placental tissue produces more hormone. Some fertility medications contain hCG and will affect results directly. Less commonly, elevated hCG outside pregnancy relates to other conditions requiring investigation.

Lower-than-expected levels in early pregnancy most often reflect dates being different from what was assumed. They can also indicate a pregnancy developing outside the uterus, or one that is not progressing — both of which need assessment rather than interpretation from a number.

What hCG won't tell you

• It doesn't assess pregnancy health. Ultrasound does that. hCG is one piece of information alongside it.
• A single value means little. The trend across samples carries the information, not one figure.
• Ranges overlap enormously. Healthy pregnancies vary by an order of magnitude at the same stage.
• It can't be influenced. Levels are determined by the placenta and no lifestyle change alters them.

If something isn't as expected

An unexpected result is a reason to speak to a doctor or midwife promptly rather than to search for an interpretation. Many variations turn out to be nothing, and the ones that matter need proper assessment quickly.

In Australia, Pregnancy, Birth and Baby offers free advice by phone and online. In New Zealand, your midwife, GP or Healthline can advise. Support is also available through pregnancy loss organisations in both countries if you need it.

The bottom line

hCG confirms pregnancy reliably and tracks its early progression usefully. What it cannot do is tell you whether a pregnancy is healthy from a single number — that requires the trend, an ultrasound, and a clinician.