Pregnancy tests, in the strictest sense, really only test for the presence of one particular substance in the woman’s body. That substance is the hormone human Chorionic Gonadotropin, which is also sometimes referred to as hCG. HCG is a hormone that a woman’s body produces after conception. Specifically, hCG is produced by the woman’s body within 1 to 2 days after the fertilized egg implants into the woman’s uterus. This can actually take place around 7-10 days after the woman ovulates. Depending on when the woman had sexual intercourse, it is possible that her body would not start producing hCG until as many as 15 days after sexual intercourse.
Pregnancy tests, in general, contain a specific chemical that will reach with the hCG that is contained in a woman’s urine. Some pregnancy tests are more sensitive to the levels of hCG than other pregnancy tests are. The most sensitive pregnancy tests will notice levels of hCG that are 25 mIU/ml, while the pregnancy tests that are not as sensitive will not notice hCG until it reaches the level of more like 250 mIU/ml. The more sensitive pregnancy tests tend to work better than the less sensitive pregnancy tests at being able to detect a pregnancy in those very early days of pregnancy, including those few days that follow ovulation.
By the time a woman has missed her period, any given pregnancy test is more than adequate to detect the presence of hCG. The levels of hCG in a woman’s system by that point are almost always high enough hat even the less sensitive pregnancy tests will work. While there is always the possibility of human error, either in the use of the test or in the interpretation of the results, for the most part pregnancy tests are a reliable indicator of whether a woman is pregnant or not, and are thought to have a success rate that is above 97% in studies that have been conducted.