There are so many factors that influence a false positive (or as muchas you don't want to believe it, sexual abuse or stuff going on when you aren't around). I just saw a special on TV that went into false positives. The urine tests are not always accuate, but if your house has any type of chemicals in it, rat droppings or something out of the ordinary, it is possible. A cyst could also make show a false positive....here's more

1. Improper testing

Improper testing can lead to false positive results. When taking a home pregnancy test, it is very important to read the directions before testing. Your home pregnancy test instructions will specify when test results should be interpreted and when test results are no longer valid. Some tests will produce a faint positive test result if read after the instructed time. You should use a clock or a watch and record the time you perform the test and read results at the instructed time. Guessing how long it has been since performing the test can lead to false results. Three minutes can seem like twenty minutes and vice versa when you are anxiously waiting to find out if you are expecting.

Another problem that commonly occurs when it come to testing is going back to "double check" your results. Disappointed with a negative result at the 3 minute (or 5 minute interval depending on the test), a woman may go back to look again to see if maybe she didn't see that positive the first time. Some women will go as far as digging that test out of the trash "just to be sure". This is probably one of the biggest causes of false positive results. Home pregnancy tests should always be read at the instructed time for accurate results.

2. Evaporation lines

Some home pregnancy tests have been described by women as having an evaporation line. If you stare at and inspect a home pregnancy test closely, it is possible with some tests to see where the testing line is. When urine runs across the test strip it may briefly change color as part of the testing process thereby helping the woman locate the test are even further. In order for the test to be positive it should change color and have a colored line at the determined testing time. With some tests, you will be able to see what can best be described as a watermark, much the same as you see on a US dollar bill when turned to the light. If there is no color to it or you have to grossly manipulate it to the light in order to observe a line, it could be an evaporation line. Retesting in a few days or using a digital test is recommended.

3. Home pregnancy test interference

Certain medications such as phenothiazine can interfere with test results. Women receiving hCG therapy to treat a luteal phase defect or other fertility problem should consult their doctor as to accuracy of home pregnancy tests and when testing should be performed. Occasionally urine can be contaminated with blood or gross amounts of protein leading to inaccurate results.

4. Certain medical conditions can lead to false positive results.

If you receive a positive result from a home pregnancy test it is important to follow up with your doctor for medical care. Although it is not common, certain rare medical conditions can cause positive results on a home pregnancy test. Certain forms of cancer, such as choriocarcinoma (an extremely rare type of cancer usually found in the uterus) can cause an elevation in hCG levels as well as certain types of breast, ovarian or testicular cancer leading to a false positive result.

5. Defective pregnancy test

Expired tests or faulty home pregnancy tests can also lead to false positive results. Always check the expiration date before testing to assure accurate results. Keep in mind, even with the highest quality control, an occasional defective test may occur.