The earliest way to know for sure whether you are pregnant is to have a pregnancy test. The fertilized egg at about 4 days old begins to secrete a hormone called human chorionic gonadotropin (HCG). Initially, it can be detected in the mother's blood, and shortly thereafter it is detected in the mother's urine.
Increased urinary levels of HCG form the basis of most tests for pregnancy and trophoblastic tumors in men. HCG is present in both blood and urine whenever there is living chorionic/placental tissue.
HCG can be further identified as alpha or beta HCG. It can be detected in the urine of pregnant women 26 to 36 days after the first day of the last menstrual period, or 8 to 10 days after conception.
Most pregnancy tests are conducted with a sample of urine, which is obtained more easily than a sample of blood. Most depend on a reaction between HCG and an antibody to HCG. A second reaction is then needed to determine whether the first reaction has taken place. Often that is a color change.
About three-fourths of the time, results of a urine test done in a clinic or a physician's office will be positive if you are pregnant and your period is 4 to 7 days late. When your period is 2 weeks late, the accuracy is close to 100 percent.