How pregnancy tests work All home pregnancy tests detect the ‘pregnancy hormone’, human Chorionic Gonadotrophin (hCG) in urine. In pregnancy, hCG is produced first by the fertilised egg and later the placenta. A fertilised egg typically implants into the lining of the uterus (womb) 6-8 days before the missed period, when tiny amounts of hCG start to appear in the mother’s blood and urine. Though hCG levels are very low at first, the level rises rapidly and predictably in the first weeks of pregnancy.
