N°004 · Pregnancy testNot medical advice

How a pregnancy test works

The pink line on many pregnancy tests is made of gold particles. Here’s how it works.

A home pregnancy test looks for hCG, a hormone made once an embryo attaches to the uterus wall. Urine moves along a paper strip by capillary action, carrying antibodies tagged with gold specks about 40 nanometres wide. Where hCG links them to the test line, the gold piles up into a pink line.

How a pregnancy test works

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A paper strip, a few drops of urine and, in many tests, specks of real gold. Here’s how they reveal one pregnancy hormone.

The hormone it looks for

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hCG (human chorionic gonadotropin) is made by the early placenta once the embryo attaches to the uterus wall, 6 to 12 days after fertilisation. Some passes into urine.

What’s inside the stick

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Typically four parts in a row: one soaks up urine, one stores dried gold particles, a membrane holds two invisible lines, and an end pad absorbs the rest.

How urine moves by itself

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No pump. Capillary action, the pull that lets a paper towel soak up a spill, draws urine along the strip and sweeps the dried gold with it.

Where the pink comes from

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Gold specks about 40 nanometres (millionths of a millimetre) wide carry antibodies, proteins that grab one target. Specks this small absorb green light, so a line of them looks pink.

How the test line forms

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hCG has two parts. Gold-tagged antibodies grab one; antibodies fixed on the test line grab the other. Only hCG can link both, so gold piles up there.

What the control line shows

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A second line holds antibodies that catch the gold-tagged antibodies themselves, hormone or not. It shows enough urine flowed past the test line. No control line: invalid test.

Why testing too early can fail

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Many tests react at 10 to 25 milli-units of hCG per millilitre. On the first day of a missed period, 10 to 20 in 100 pregnant women test negative.

Before strips: mice, rabbits, frogs

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From the late 1920s, labs injected urine into mice, rabbits or frogs. Home kits arrived in the 1970s and took two hours. Today’s strips take minutes.

So that’s how.

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Not medical advice

Health topics are explained for general information only. This is not medical advice: talk to a doctor or pharmacist about your own situation.

Not medical advice. The test shown is a generic design.

Devices shown are generic illustrations, not real products. Not sponsored.

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Made with the help of AI tools and checked against primary sources. The videos carry Content Credentials (C2PA).

Sources

Slide 1 of 10

  1. In many tests the pink line is made of gold particles ("colloidal gold conjugate", "form a pink line")accessdata.fda.gov

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  1. hCG (human chorionic gonadotropin) is made by the early placenta after the embryo attaches; it first appears 6 to 12 days after ovulation; some passes into urinepubmed.ncbi.nlm.nih.gov
  2. hCG (human chorionic gonadotropin) is made by the early placenta after the embryo attaches; it first appears 6 to 12 days after ovulation; some passes into urinesciencedaily.com
  3. hCG (human chorionic gonadotropin) is made by the early placenta after the embryo attaches; it first appears 6 to 12 days after ovulation; some passes into urinefda.gov
  4. The embryo attaches by the side that carries the inner cell massfrontiersin.org

Slide 3 of 10

  1. A typical strip has four parts in a row: sample pad, conjugate (gold) pad, nitrocellulose membrane with the T and C lines, end (absorbent) pad, on a plastic backingmdpi.com
  2. A typical strip has four parts in a row: sample pad, conjugate (gold) pad, nitrocellulose membrane with the T and C lines, end (absorbent) pad, on a plastic backingcdn.shopify.com

Slide 4 of 10

  1. No pump: capillary flow moves the sample and the dried gold; most tests are read after about 3 to 5 minutes (follow the leaflet)accessdata.fda.gov
  2. No pump: capillary flow moves the sample and the dried gold; most tests are read after about 3 to 5 minutes (follow the leaflet)fda.gov

Slide 5 of 10

  1. Gold particles about 40 nm wide carry antibodies; at that size they absorb green light (peak about 520 to 530 nm), so a line of them looks pinksigmaaldrich.com
  2. Gold particles about 40 nm wide carry antibodies; at that size they absorb green light (peak about 520 to 530 nm), so a line of them looks pinkcdn.shopify.com

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  1. Gold-tagged antibody grabs the beta part of hCG; the antibody fixed on the test line grabs the alpha part; the alpha part is also in LH, FSH and TSHaccessdata.fda.gov
  2. Gold-tagged antibody grabs the beta part of hCG; the antibody fixed on the test line grabs the alpha part; the alpha part is also in LH, FSH and TSHncbi.nlm.nih.gov

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  1. The control line catches the gold-tagged antibodies themselves; it shows enough sample flowed past the test zone ("adequate sample volume and proper wicking"); no control line means an invalid testaccessdata.fda.gov
  2. The control line catches the gold-tagged antibodies themselves; it shows enough sample flowed past the test zone ("adequate sample volume and proper wicking"); no control line means an invalid testaccessdata.fda.gov

Slide 8 of 10

  1. Many tests react at 10 to 25 mIU/mL; on the first day of a missed period, 10 to 20 in 100 pregnant women test negativeaccessdata.fda.gov
  2. Many tests react at 10 to 25 mIU/mL; on the first day of a missed period, 10 to 20 in 100 pregnant women test negativeaccessdata.fda.gov
  3. Many tests react at 10 to 25 mIU/mL; on the first day of a missed period, 10 to 20 in 100 pregnant women test negativefda.gov
  4. Many tests react at 10 to 25 mIU/mL; on the first day of a missed period, 10 to 20 in 100 pregnant women test negativescholars.duke.edu
  5. The hCG curves are schematic (implantation timing varies); the axis counts days after ovulationpubmed.ncbi.nlm.nih.gov

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  1. Mouse test from the late 1920s, then rabbits and frogs (frogs laid eggs 8 to 12 hours later); "the rabbit died" whatever the result; home kits in the 1970s took about two hours; the 1988 one-step stick took three minuteshistory.nih.gov
  2. Mouse test from the late 1920s, then rabbits and frogs (frogs laid eggs 8 to 12 hours later); "the rabbit died" whatever the result; home kits in the 1970s took about two hours; the 1988 one-step stick took three minutesijdb.ehu.eus
  3. Mouse test from the late 1920s, then rabbits and frogs (frogs laid eggs 8 to 12 hours later); "the rabbit died" whatever the result; home kits in the 1970s took about two hours; the 1988 one-step stick took three minutesogmagazine.org.au
  4. Mouse test from the late 1920s, then rabbits and frogs (frogs laid eggs 8 to 12 hours later); "the rabbit died" whatever the result; home kits in the 1970s took about two hours; the 1988 one-step stick took three minutessmithsonianmag.com
  5. Mouse test from the late 1920s, then rabbits and frogs (frogs laid eggs 8 to 12 hours later); "the rabbit died" whatever the result; home kits in the 1970s took about two hours; the 1988 one-step stick took three minutescollection.sciencemuseumgroup.org.uk