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How does a pregnancy test work?

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Quick answer

A pregnancy test works by detecting human chorionic gonadotropin (hCG), a hormone produced by the placenta after a fertilised egg implants in the uterus. The test strip contains antibodies that bind to hCG and produce a visible line or digital reading if the hormone is present in urine.

A pregnancy test detects human chorionic gonadotropin (hCG), a hormone the placenta produces after a fertilised egg implants in the uterus. The test uses antibodies that bind hCG and trigger a visible colour line or digital reading.

Step by step

  1. A fertilised egg implants in the uterus lining, and the placenta begins to form.
  2. The placenta releases hCG into the bloodstream, which filters into urine.
  3. hCG in urine binds to specific antibodies on the test strip.
  4. The bound hCG-antibody complexes migrate along the strip and attach to a second set of antibodies.
  5. This attachment concentrates coloured particles, forming a visible line or triggering a digital signal.

Key things to understand

  1. After fertilisation, the developing embryo implants in the uterus lining and the placenta begins forming.
  2. The placenta releases human chorionic gonadotropin (hCG) into the bloodstream, which then appears in urine.
  3. hCG levels rise quickly in early pregnancy, roughly doubling every two to three days at first.
  4. A test strip contains antibodies that bind specifically to hCG, often with a dye attached.
  5. When hCG is present, it links the antibodies and creates a visible coloured line or digital signal.

Labelled diagram

  1. Sample pad

    Urine is applied here and wicks into the test strip.

  2. Conjugate pad

    Contains antibodies linked to coloured particles that bind hCG.

  3. Test line

    Fixed antibodies capture the hCG-antibody complexes, forming a line.

  4. Control line

    Captures excess antibodies to show the test worked correctly.

  5. Result window

    Lines or digital readout show whether hCG was detected.

How a pregnancy test detects hCG

Real-world examples

  • A person who misses a period uses a home test kit, holds the stick in urine, and sees two lines appear within minutes.
  • A doctor orders a blood test to measure the exact level of hCG, which can detect pregnancy earlier than a urine test.
  • In a clinic, a urine sample is tested with a dipstick that changes colour to indicate the presence of hCG.
  • A digital test displays the word 'pregnant' or 'not pregnant' based on the same antibody reaction.

Common mistake

✗ Myth: A pregnancy test can detect pregnancy immediately after sex.

✓ Fact: It takes time for an embryo to implant and for the placenta to start producing hCG. Most tests are accurate only after a missed period, roughly two weeks after conception.

People also ask

What does a pregnancy test detect?
A pregnancy test detects human chorionic gonadotropin (hCG), a hormone produced by the placenta after a fertilised egg implants in the uterus. hCG appears in urine and blood, and its presence indicates pregnancy. The test uses antibodies that bind specifically to hCG and create a visible signal.
How accurate are home pregnancy tests?
Home pregnancy tests are about 99% accurate when used correctly and after a missed period. Testing too early or not following instructions can lower accuracy. Factors like diluted urine or certain medications can also affect results.
Can you get a false negative pregnancy test?
Yes, a false negative can happen if you test too early, before hCG levels are high enough. Using diluted urine or a test that has expired can also cause a false negative. Waiting a few days and testing again usually gives a clearer result.
How soon after conception can a pregnancy test detect hCG?
Most tests can detect hCG around the time of a missed period, typically about two weeks after conception. Implantation usually occurs six to twelve days after fertilisation, and it takes a few more days for hCG to reach detectable levels in urine.
Why do some pregnancy tests show a faint line?
A faint line usually means hCG is present but at a low concentration, which is common very early in pregnancy. Testing again after a few days often gives a darker line as hCG levels rise.
Can a pregnancy test be positive if you are not pregnant?
False positives are rare but can happen. They may be caused by certain medications containing hCG, recent miscarriage or abortion, or medical conditions that produce hCG. A doctor can confirm with a blood test.
Do pregnancy tests work the same for everyone?
The basic mechanism is the same, but factors like the timing of ovulation, the sensitivity of the test, and how diluted your urine is can affect results. Testing first thing in the morning gives the most concentrated urine.

Part of the guide: How Your Body Works →