It depends on you and your blood type. one of my friends had a blood test and it was negative because her levels were not high enough to read as a pregnancy. So if your hormornes dont read a certain number they say you not pregnant
it has happen to me and I have a 17 month old daughter. I had a blood test and they say I was not pregnant 2 months later I was 8 weeks from the time they said i was not pregnant do the math