conception occurs (on average) at 2 weeks pregnant. this is 2 weeks after the start of your period. the fertilized egg doesn't implant until 5 days after conception. so almost at week 3. your body doesn't start making pregnancy horemones until the egg implants and you actually become pregnant.
so testing at 3 weeks pregnant means your body has been making horemones for 2 days. there is no way a home pregnancy test will detect the pregnancy.
So if you are counting from the first day of your missed period, wait a week or 2 and test again.
at 10:49 AM on Aug. 8, 2012