Extracted from BabyHopes

When used properly, and at the right time, home pregnancy tests are very accurate. In fact, is is believed that home pregnancy tests may be as much as 99% accurate when they show a positive result. Having said that, there are some important factors to think about when considering exactly how accurate home pregnancy tests are.

It helps to understand a little bit about how pregnancy occurs, and about how home pregnancy tests work in order to understand how accurate they are. Home pregnancy tests check for the presence of a particular hormone known as human Chorionic Gonadotropin, or hCG as it is called for short. Some home pregnancy tests are more sensitive than others. That is, they will detect a lower presence of hCG than the less sensitive tests will. The most sensitive of the home pregnancy tests will detect hCG at around 25 mIU/ml, whereas the least sensitive home pregnancy tests will not detect hCG levels until they get to be 250 mIU/ml or more.

When an egg becomes fertilized, a woman’s body does not begin to produce hCG right away. In fact, it is usually between 7 and 10 days after a woman ovulates that the fertilized egg will implant in the uterus. This is the time that the woman’s body will start to produce hCG. Within 1-2 days of implantation, there is typically enough hCG in the woman’s body for the most sensitive of the home pregnancy tests to give a positive result. If you use a less sensitive test, and if it is used too early in the pregnancy, it may give you a false negative result because there just isn’t enough of a concentration of hCG yet for the test to be able to register it.

To get the most accurate results from a home pregnancy test, you will typically want to not use the test until you have actually missed your period. At that point, even the least sensitive tests will generally be able to detect the pregnancy accurately.