For the most accurate results, health professionals advise that you take a pregnancy test one week after your first missed period. Taking a pregnancy test on the first day of your missed period can also be reliable.
You should also know that some medications or hormonal conditions may interfere with the accuracy of a pregnancy test. Also, it is possible to have a period and to be pregnant, most common with women who use hormonal contraceptives as the hormones can mask the natural effects of pregnancy. If you have had unprotected sex and then experience a change in your period, i.e. it is lighter than usual, this could be an indication that you are pregnant, so you may wish to take a pregnancy test to be sure.
Pregnancy tests rely upon detecting a hormone called HCG. The levels of HCG can take a while before they are high enough to be detected by a pregnancy test. A home urine test with high sensitivity could show an accurate result within 2-3 days of fertilisation, but low sensitivity tests may not produce an accurate result until 6-7 days after implantation.
Many home pregnancy tests claim to be 99% accurate if used correctly, but reliability can be reduced in some tests when women take them earlier. Health professionals recommend that you wait until one week after your first missed period, for more reliable results.
A positive pregnancy test is most likely to be accurate; however, a negative result is less reliable. For the most accurate results, you should take a pregnancy test first thing in the morning, when your urine is of the highest HCG concentration. If you take a pregnancy test incorrectly, you may also get a false-negative result (when the result comes up as negative but should be positive), so ensure that you always read the instructions carefully before use. If you realise you have made a mistake with your test, you should take another test to be sure.
If you want to know sooner if you are pregnant, or if you want the most accurate result, you should be able to arrange a pregnancy blood test with your doctor. A blood test is the most reliable way to find out if you are pregnant or not, and if you are pregnant, you should be able to find out how far along you are in your pregnancy.