The fertilized egg enters the uterine cavity within 4 days of conception and is considered the beginning of pregnancy from ovulation to the union of egg and sperm. The location of the union of sperm and egg is in the abdomen of the fallopian tube, and the union of sperm and egg is called fertilized egg. The fertilized egg relies on the peristalsis of the fallopian tube and the oscillation of the mucous membrane of the fallopian tube to move into the uterine cavity. On day 5-6, the zona pellucida of the early blastocyst disappears, increases in size and develops into a late blastocyst. The process of gradual implantation into the endometrium on day 6-7 and its covering by the endometrium is called implantation. After implantation, gradual development begins until it develops into a fetus.