A home pregnancy test looks for human chorionic gonadotropin, or hCG, in urine. The hormone usually rises after implantation. The test is designed to show whether hCG is present above its detection threshold, rather than measure every detail of a pregnancy.
The test strip and control
Urine travels along a test strip and encounters reagents that bind hCG. The visible result depends on the specific test design; a control indicator shows that the liquid moved through the device as intended. The package instructions determine how long to wait and when a result should be read.
Why timing changes the answer
Testing too early can give a negative result even if hCG later rises enough to detect. Dilute urine, expired tests, and reading outside the stated time can also confuse interpretation. A positive home result is a reason to contact a healthcare professional for next steps; the strip cannot locate a pregnancy or assess its health.
Related: how a continuous glucose monitor measures a different biomarker.