How Does a Pregnancy Test Work?
A-Level Biology Explained
Pregnancy tests look deceptively simple: a plastic stick, a couple of lines, and a life-changing result.
But behind that little window is some very elegant immunology that fits beautifully into A-Level Biology.
Let’s break it down.
1️⃣ The Key Hormone: hCG
Pregnancy tests detect human chorionic gonadotropin (hCG).
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hCG is a glycoprotein hormone
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Produced by the trophoblast cells of the embryo
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Appears in blood and urine shortly after implantation
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Its role is to maintain the corpus luteum, keeping progesterone levels high so the uterine lining isn’t shed
➡️ No implantation = no hCG = negative test
2️⃣ Pregnancy Tests Are Lateral Flow Immunoassays
A pregnancy test works in the same basic way as:
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COVID lateral flow tests
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Some drug tests
Urine moves along the strip by capillary action.
Inside the test strip are:
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Mobile antibodies (attached to coloured beads)
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Fixed antibodies (anchored in specific zones)
3️⃣ Step-by-Step: What Happens in the Test?
🟡 Step 1: Sample Application
Urine is applied to the absorbent pad.
If hCG is present, it binds to mobile monoclonal antibodies tagged with coloured particles.
🔵 Step 2: The Test Line
The hCG–antibody complex moves along the strip and reaches the test line.
Here:
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A second antibody (specific to hCG) is fixed
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This traps the complex
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The coloured particles accumulate → visible line
✔️ This is a positive result
🔴 Step 3: The Control Line
Excess antibodies continue moving to the control line.
This line:
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Always appears if the test is working
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Confirms proper flow and reagent function
❗ No control line = invalid test
4️⃣ Why Timing Matters
Early testing can give a false negative because:
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hCG concentration is still low
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Detection threshold hasn’t been reached
This is why:
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Tests recommend first-morning urine
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Waiting until after a missed period improves accuracy
➡️ Great exam link to concentration, sensitivity, and specificity
5️⃣ Exam Gold: Key A-Level Biology Concepts
This topic links beautifully to:
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Monoclonal antibodies
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Antigen–antibody specificity
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Hormonal control in pregnancy
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Cell signalling
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Biotechnology in medicine
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Ethical and social implications of testing
Perfect for:
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4–6 mark explanations
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Data interpretation questions
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Synoptic essays
One-Sentence Exam Summary
Pregnancy tests work by using monoclonal antibodies in a lateral flow assay to detect the hormone hCG in urine, producing a visible line when antigen–antibody complexes form.
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