How can a pregnancy test tell you how far along you are?

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I mean, what is it within the fluids that are used to test for a positive/negative pregnancy that is able to tell you how mature the fetus is? Is it something that only becomes existent once the baby starts growing or is it an indication from a particular organ? Thanks!

An at home test cannot determine how far along you are. Only a doctor would be able to based on your last menstrual cycle.
i did a clear blue digital test and it cld tell me i was 3 weeks ,
No.
Only a doctor and ultrasound can determine how old the baby is.
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