system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How does a pregnancy test work? Especially how does the first stripe appear almost immediately, but the 2nd only after 15min?
gpt
When a women becomes pregnant, her body starts produced tons of hormones to prepare her body to sustain and carry the child. Some of these hormones will begin to collect in the woman's urine with days or weeks of fertilization.

The tests work by triggering a two phase chemical reaction. The first reaction basically just detects water, that's the first, "not pregnant" line. The second stripe contains a chemical that changes color when it reacts with the hormones, but it takes more time. Think of it this way, there's a ton of water in pee so that's easy to detect, there's only a tiny bit of hormones so they're harder to find.

So you'll always get that first stripe, if you don't it's a sign the test is defective. The second line, if it appears, takes a while because it's the part that's actually "doing something".