The principle of the test pen is to detect the "progesterone" in the urine, that is, human chorionic gonadotropin (hCG). During pregnancy, hCG is first produced by the fertilized egg and then by the placenta. The fertilized egg is usually implanted in the endometrium (uterus) 6-8 days before the expected menstrual delay, at which point traces of hCG begin to appear in the mother's blood and urine. Although hCG levels start out very low, they rise quickly and predictably during the first few weeks of pregnancy.