Pregnancy occurs after sexual intercourse. The male's penis is inserted into the female's vagina, and semen (the fluid which contains sperm) is ejaculated in or near the vagina.
The sperm swim up through the cervix (the bottom opening into the uterus) and meet the egg that has been released by an ovary. A single sperm joins with the egg and pregnancy begins.
The fertilized egg travels down the mother's fallopian tube about seven to 10 days after fertilization and burrows into the wall of the uterus. Until this occurs, the mother is unaware of the fertilized egg, and tests cannot detect the pregnancy.
Tests become more reliable in detecting pregnancy about three weeks after fertilization. Home urine pregnancy tests are reliable about one day after a regular period has been missed.