It takes three to four days after fertilization for the egg to make its way down the fallopian tube toward the uterus and attach to the wall lining. This is known as implantation.
Human chorionic gonadotropin (hCG) is generally detectable in the blood about a week after implantation. At this point, a doctor can perform a pregnancy test with reasonable accuracy.
Some home pregnancy tests claim to be able to detect pregnancy five days before a missed period. Since the next period usually occurs 14 days after ovulation, a home pregnancy test can read positive as early as five or six days after implantation.