well your body doesn't know it is pregnant until about a week after conception. Even then, the hormone levels are too low usually for a hpt test to pick it up. You can test 5 days before your missed pd, but even then it may not be accurate. The best time would be the day you missed. If it shows up neg, wait a few days and test again. I think the hormone levels double everyday, so the further along you are, the more hormones the test has to work with.
at 7:08 PM on Dec. 14, 2009