Hi S.,
This is a subject I happen to know a little about, as I had to do a study on it for a college course several years ago. Although false positives on pregnancy tests can happen, they are VERY rare. Here are a couple of reasons for a false positive:
Chemical Pregnancy: this is basically a miscarriage. Research has shown that half of all pregnancies end in miscarriage. It is possible to take the test, show a positive, and miscarry within the next few days causing a positive test result. Since this happens within the first month of pregnancy, the cells do not even have a chance to attach to the uterus, and are excreted during your menstrual cycle.
Chemical Interference: There are drugs that have the Human Chorionic Gonadotropin hormone in them (hCG- the hormone produced in the body during pregnancy). If you have taken any medications with this hormone, it will cause a false pregnancy test.
Reaction time: If you let the test set for over ten minutes after taking it, it can cause the test to be invalid and show a false positive. You need to read the test around 3-5 minutes after taking.
The best bet is to get a test that reads "Pregnant" or "Not Pregnant" - If you received a positive test result, most likely you are pregnant, but as I indicated earlier, there are some instances that may interfere causing a false positive test - but again, very rare. Let me go ahead and extend my congratulatory sentiments now : ) !!
Good Luck S.!
A.