Is it true that some women never test positive on an hpt, their whole pregnancy? or until they are like 20 weeks? what is the latest someone could receive a false negative?

Always positive: the whole time of the pregnancy, and for a few weeks after it completes.

No. No.: Today's home pregnancy tests are very accurate and not affected by foods, herbs, common illnesses, sexually transmitted diseases, vaginal spotting, or medications. A test gives a reliable result by 3 weeks after sex (3 weeks after when conception might have been) or 1 week after missing a period. If done sooner, a negative result may be due to testing too soon, such as less than 2 weeks after sex.