Over-the-counter pregnancy tests claim to be accurate on the first day of a missed period — or even before. You're likely to get more accurate results, however, if you wait until after the first day of your missed period.

Why wait? Shortly after a fertilized egg attaches to your uterine lining (implantation), the placenta forms and produces the hormone human chorionic gonadotropin (HCG). This hormone enters your bloodstream and urine.

During early pregnancy, HCG concentration increases rapidly — doubling every two to three days. The earlier you take a home pregnancy test, the harder it might be for the test to detect HCG.

Fertility drugs or other medications that contain HCG might interfere with home pregnancy test results. However, most medications, including antibiotics and birth control pills, don't affect the accuracy of home pregnancy tests.

Keep in mind that the timing of your ovulation might vary from month to month, and the fertilized egg can implant in the uterus at different times. This can affect the timing of HCG production and when it can be detected. If your periods are irregular, you might miscalculate when your period is due.

If it's important to confirm your pregnancy right away and depending on how far along you are in your pregnancy, your health care provider might recommend that you have an ultrasound, repeat a urine test at a lab or have a blood test to measure your HCG levels.