To support the ongoing pregnancy every Pregnant Women produces a hormone called hCG (Human Chorionic Gonadotrophin). The pregnancy test kits work by detecting this hCG in the urine of the pregnant women. Usually hCG appears in the urine of pregnant women after 20-24 days from the day of her last menstrual period. The level of hCG rapidly increases (almost doubles every 72hrs) till the third month of pregnancy and then maintains a steady state.
Now coming to the question how pregnancy test kits detect hCG.
The pregnancy test kits will have a chromatographic paper enclosed inside a plastic case and you can see the chromatographic paper from the upper side. The Chromatographic paper will be impregnated with hCG and anti hCG (a substance against hCG) at the point marked as ‘C’ and only with anti hCG at the point marked as ‘T’. C stands for “Control” and T stands for “Test”.
Am I Pregnant?
There will a sample well where urine drops has to be poured and the urine will blot through out the chromatographic paper
Now the principle is
hCG + anti hCG + water = color
Let us take an example that a woman is not pregnant and she wants to check whether she is pregnant. She will put her urine in the sample well and the urine will blot through the chromatographic paper and it will cross both the “control” as well as the “Test” bands.
Now apply the principle here
Since the women is not pregnant her urine will not have hCG
At the Control area which is already impregnates with hCG +anti hCG a color band will appear because of the water from the urine but at the test area which is having only anti hCG no color will appear
so What happens at Control Band is?
hCG +anti hCG +water (from urine) = color.
This is basically to ensure that the pregnancy test kit is working.
and What happens at Test Band is?
Anti hCG + water (from urine) = no color
Since the woman is not pregnant her urine will not have hCG. There will be only one pink colored band appearing at the control area and the test band will not be colored.
Let us assume that the woman is pregnant and her urine will have hCG now.
Applying the principle
At Control Band
hCG (already impregnated) + hCG (from urine) + anti hCG + water (from urine) = color
At Test Band
hCG (from urine) + anti hCG + water (from urine) = color
Now two colored bands can be seen at both Control and Test areas.
This means the test is positive. Congrats you are pregnant and it's time for you to consult a Doctor.
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