Does every women get a positive hpt if she is pregnant?

Depends. On which test you use and when you take the test. Most tests will be close to 99% accurate by a week after your missed period. If you do the test on the day you miss your period, when you first get up in the morning, if you are pregnant the test will only be positive approximately 80% of the time. Here's a link to a good article to read about this: http://bit.Ly/rgb1xs.