Never ending worries!

So I had my ultrasound last Thursday when I was 12+6, and baby measured 2.5 in long and the tech said the baby's size was perfect but everything I'm reading for this week says baby is 3 in long at this point and I can't stop worrying that my baby's not growing fast enough and that this is a sign that something is wrong. I know it's totally irrational, but I guess that's part of pregnancy, right?