Good luck, MrsP! Hope the scan goes well. I doubt you will be able to determine gender without someone pointing it out. I often had to be told which end was baby's head! Those ultrasounds are tricky sometimes.
Measurements are also all over the place. Some women just have small babies. My ultrasound the day before Gabby's birthday put baby at 7 lbs, and she was born 6 lbs 13 oz, so it was pretty close, but then you have other people who are told they are having giants and the baby is born small So its all over the place, really.
Gabby has not been napping well yesterday or today, so I'm one exhausted mommy! Overtired babies are no fun at all!