Pregnancy tests detect the presence of Human Chorionic Gonadotropin (HCG) in your urine. Your body produces HCG in response to a fertilised egg attaching itself to the wall of your uterus. This usually takes place around six days after fertilisation. The more sensitive pregnancy tests can detect very low levels of HCG as early as four days before your period is due. However, if you test too early, even with a sensitive test, the level of HCG in your urine may not be high enough to detect. Most pregnancy tests will give up to 99% accuracy if you test around the time your period is due (about two weeks post ovulation).