How common is it to test Negative but be Positive?

  Is it fairly common for a woman to be tested for pregnancy, and have the result be negative, but then weeks later test positive?  My period is now 10 days late, and it's never been late before.  I've taken two home tests (the day after and then 3 days after that) and then 2 days ago I took a blood test- all negative.  I still have no signs of starting my period!  I've heard that in some cases the tests aren't accurate- but how common is that?  When should I become concerned about not having a period??
Dear Lisa:
Please see earlier questions with the same topic.
It is very uncommon for a pregnancy test to be negative and then become positive. This only happens when the day of ovulation has moved and the testing was done before pregnancy would be detected. When three pregnancy tests, as you describe are negative, one must consider other causes for missing a menstrual period. The most common is that normal women do this randomly and without explanation.
Keywords: pregnancy test
This information is provided for education purposes only and is not a medical consultation. If you have specific questons, please contact your physician.