If egg is fertilized by sperm then this fertilized egg has to travel from fallopian tube to uterus for implantation. This journey can take some time that can be of 7-10 days. If a woman has 28 days cycle and ovulates at mid of the cycle then she can conceive within 24 hours of ovulation. The fertilized egg will reach to uterus on 23-25 day of cycle. At this time some women can have implantation bleeding. Pregnancy test at this time might be false negative but can become positive after few more days especially on due date of period.
