The pregnancy tests available on the market today can range widely in their sensitivity in picking up the pregnancy hormone Human Chorionic Gonadotropin (better known as hCG). This is why it is so important to know which tests are able to detect hCG sooner then others.
The hCG hormone is what the pregnancy test is sensitive to. By using a more sensitive pregnancy test, this allows you to detect earlier if your attempts to conceive were sucessful and to recieve proper medical care through your health care professional.
Unfortunately, there is a down side to extra sensitive pregnancy tests as about 25% of all pregnancies sadly end in miscarriage and many of these being very early pregnancies. If you were to use a very sensitive pregnancy test, which is where you would test the day your period is due, you could catch an early pregnancy but end up going on to have your period anyway due to a very early miscarriage.
The hCG hormone is measured in milli-international units per milliliter ,(mIU). Pregnancy tests that are 25 mIU are generally more sensitive to picking up the pregnancy hormone compared to a test that is 50 mIU. You must use caution in making too much of hCG numbers because sometimes a normal pregnancy will have low hCG levels and result in a perfectly healthy baby. However, a low hCG level can also mean any number of things and should be rechecked within 48-72 hours to see how the level is changing.
hCG levels can be detected about 12-14 days after conception by a urine test. Also please remember that hCG levels can vary dramatically from one pregnancy to another.