So, how can the home pregnancy tests claim 99% accuracy? Aren't they regulated by the Food and Drug Administration (FDA)? They can say the 99% because of an outdated FDA rule that says that the test needs to agree with those of an existing test more than 99% of the time. This is easy for the newer tests to do, since they are better, but they are not 99% accurate, they just agree with the older, inaccurate tests 99% of the time.

Some of the fine print in the packaging clarifies a little, but still the overall message is very misleading, making it easy to put too much confidence in the result.

An article in the American Journal of Obstetrics and Gynecology (Volume 190, Issue 1 , January 2004, Pages 100-105) also noted an alarmingly high false positives in a few brands: Confirm (1 out of 6 was a false positive) and Clear Choice HPTs (3 out of 6 were falsely positive: 50%!). The most reliable tests were First Response, Early Results, followed closely by Clear Blue Easy, One Minute and Clear Plan Easy.

Some of the problems with the validity claims have to do with the wide range of hcg (which is what shows up in your urine when you are pregnant) that a pregnant woman can produce. Click here for a chart of the range of hcg at different days after ovulation. The best home pregnancy tests can detect 25mIU/mL in your urine, and the range goes up with some tests detecting 100mIU/mL.

Everyone likes to tell you that if the urine pregnancy test is negative but you haven't gotten your period to wait a week and re-take the test. This advice is for those with the patience of a saint. Mere mortals may go ahead and test every day, at least once a day until your get a positive result or your period, or you go to the doctor.