Just wondering how accurate is a ultrasound for dating a pregnancy at 13 weeks? And what thee margin or error is.

They aren't exact, but at that stage there is roughly a several day window either side. Early on, babies follow a pretty uniform pattern of development/measurements This becomes less so later in pregnancy when ultrasound is less reliable for sizing.

You won't, for example, be off by weeks at 13 weeks. It is also important to remember that the date you get from an ultrasound will most often be different from what you would get by using your last period to date a pregnancy. Ultrasound would be the more accurate means for dating.

If you are looking for a due date or a conception date? For conception date you will still have a plus or minus 2 week window. They go by average size and development of the fetus. So if you are more or less advanced from average it can alter dates. What are you trying to determine from this?

I was just curious because my actual lmp was oct 27 2011 but my ultrasound lmp was oct 18 2011? So I didn't know if the ultrasound was off or if I just conceived 2 days after my period.