The fertilised egg, now known as an embryo, develops in the fallopian tube for the first three days, then travels down into the uterus. By the fifth day it will become a blastocyst, a hollow ball of cells surrounding a cyst-like cavity.
Once the blastocyst breaks free from its shell, or hatches, it is ready to adhere to the surface of the endometrium. It begins to secrete human chorionic gonadotrophin (hCG), a hormone that tells the corpus luteum to continue progesterone production. A home pregnancy test will detect hCG by Day 28 of a menstrual cycle.