Ok so I'm 35 weeks and 5 days pregnant now, my due date is march 11, 2013, I found out I was pregnant when I was 6 weeks and they worked out that the conception was June 4th. Now me and my partner had broken up but I slept with my ex on may 8th, but now I'm back with my partner, please tell me that there is no way it's my exs baby, just need some piece of mind

I would like to give you peace of mind, but I can't. The only way you will know beyond the shadow of a doubt is to have a DNA test done after the baby is born.

Well to help you out a bit more me and my ex only slept together once and he pulled out, a week later me and my partner had sex almost everyday. If it was my exs baby I would be 39 weeks pregnant now and my scans all say I'm 35 weeks pregnant.
Can they be very wrong?

All of pregnancy and conception is a guessing game. An educated guess if you will. Since there is no camera in the fallopian tube to know exactly when the egg is fertilized, we have formulated a mathematical calculation to determine due dates. That calculation is the same for every woman, however not every woman has the same length cycles. So, there is a margin for error. Every pregnancy also has approximately a two week window either way.