Clare, I can't begin to imagine what you are going through.
they are probably going off your LMP date, as most medical professionals I have delat with don't like the notion of charting or the we might actually know when we ovulate

Going on this info you have it would seem that your baby is growing properly, just they have your dates wrong.
Please know that you are in my thoughts. Good luck tomorrow and/or in 3 weeks time.