What does it mean when you're 31weeks and the arms and legs are measuring 2weeks behind at sonogram screening

Not much: Because you are 3/4 of the way through your pregnancy, those ultrasound measurements are less and less accurate regarding due date calculation. Now, we are entering into the arena of size/shape variability. This is probably completely normal. At this point, the most important things on ultrasound are heart, brain, kidney, and other vital organ evaluations. You have 9 weeks of booties to knit!