has anyone ever got there dates completely messed up when being pregnant?

Im not sure wether I've messed my dates right up and I possibly could have ovulated later than I thought which would make me less along than I think? Anyone else ever done this? Thinking I was 8weeks almost 9weeks and they put me down as 5-6 weeks and midwifes said I could miscarry as it may have stopped growing if my dates are correct, I have another appointment next Friday to see if there is any growth? I have my fingers crossed so much!!! Anyone else ever had this and everything been okay in the end? X