What could it mean if you should have been 7 weeks pregnant but measured 5 and a half weeks on the ultrasound?

Not really. Babies grow at pretty precise rates and therefore conception is pretty easy to answer at this early in your pregnancy. You are more likely to have your dates mixed up or that you conceived at a different time than you thought. Yes the date could be off a bit but ultrasound dates are usually pretty clear and accurate on dating embryos. Its all down to the measurements taken of the embryo and last period etc, that give a estimate of the conception date. When an ultrasound shows you are 7 weeks pregnant it actually means you are 5 weeks from conception. Pregnancy is on average 40 weeks from the start of the last period. This is because in the days before ultrasound this was the only date we knew. You don't actually conceive until 2 weeks after this date.