How Home Pregnancy Tests Work
Home pregnancy tests (HPTs) are designed to detect hCG, a hormone released by the placenta right after the embryo begins implanting into the uterine lining. The hormone is released in a pregnant woman’s urine.
An October 2001 JAMA article states the highest possible screening sensitivity for an hCG-based pregnancy test conducted on the first day of a missed period is 90 percent, as 10 percent of women may not have implanted yet. The authors estimate that the highest possible screening sensitivity of a home pregnancy test by one week after the first day of the missed period is 97 percent.