She had her period, but now after intercourse, her period is late, test is negative

I have question. My fiancé had her period after all that. But then after we had sex after that she is almost 2-1/2 weeks late and we took a home test. It said negative. And we are like 70-80% sure she is pregnant. How do we figure it out if she test failed us and she missed her period for almost 3 weeks. Or do we have to wait a little long to take the test? If any of that makes sense. Sorry if I'm making things so complicated. :/