Implantation of a fertilized blastocyst occurs between 6 and 14 days after sexual intercourse. A blastocyst is the term for a fertilized egg whose cells are rapidly dividing. Normally, sperm must reach an egg within 24 hours of ovulation because the egg won’t survive in the fallopian tubes much longer. After the egg is fertilized, it is termed a blastocyst. This blastocyst continues to rapidly divide on its way to the uterus. Once there, the blastocyst digs away a small spot in the uterus in which to attach itself. This process sometimes involves a bit of bleeding called implantation spotting. It is only when the blastocyst is firmly attached to the wall of the uterus that it begins to secrete the hormone HCG; human chorionic gonadotropin hormone. This hormone prevents the breakdown of the corpus luteum in the ovary, that stimulates progesterone to be released. Progesterone stimulates blood vessels to grow on the wall of the uterus. When taking a home pregnancy test, this is the hormone that is being tested for.