It takes 7-9 days for the sperm and the egg to unite and then to settle in the uterine cavity. After sex, some of the sperm are able to pass through the many barriers to the fallopian tube and meet the egg. When the sperm and egg unite, a fertilized egg is formed, a process that usually takes 24 hours. After the formation of the fertilized egg, the egg will gradually move slowly towards the uterine cavity and after 24 hours the first cell division will occur. On the third day of union, the fertilized egg undergoes 3-4 divisions. On the fourth day of union, the fertilized egg reaches the uterine cavity and stays there for 3-4 days before slowly adhering to the uterine mucosa and settling. The sperm and egg will gradually secrete HCG in the body 7-9 days after union, and pregnancy can be detected if a blood test is taken at this time.