Home pregnancy tests detect the presence of a hormone in a woman’s urine. This hormone, called human chorionic gonadotropin, is normally produced only during pregnancy.

A positive test result from a home pregnancy test is very reliable in confirming that a woman is pregnant. However, getting a negative test result can be less reliable. The accuracy of this test depends on when it was performed. Here’s why.

Human chorionic gonadotropin can be detected as early as one week after conception occurs. The timing of conception depends on the timing of ovulation (the release of the egg from the ovary), fertilization (the union of the egg and sperm), and implantation (when the fertilized egg affixes to the uterus). Given all these factors, it is best not to rely on a negative home pregnancy test until at least one week after the time of your expected period.

Human chorionic gonadotropin can also be measured in blood. Blood pregnancy tests are more sensitive than urine tests. If you have any question about the results of a home pregnancy test, follow up with your doctor for a blood test.