How does a pregnancy test work?

After a fertilised egg implants in the lining of the uterus, usually 6 to 12 days after ovulation, the developing placenta starts producing human chorionic gonadotropin, or hCG. Levels roughly double every two to three days in early pregnancy. Home pregnancy tests use a strip coated with antibodies that bind to hCG in urine; if enough hormone is present, a coloured line or digital result appears. Most tests claim about 99 percent accuracy from the day of a missed period, although testing too early can give a false negative. Blood tests done by doctors can detect even smaller amounts of hCG.