How do you know how big your baby is?

spainy17

Does everyone get a growth US or is that something they only do if the doc thinks there is a growth issue? 

People on here are saying things like "oh, well I am carrying a ____ pounder or my baby is measuring in the ____th precentile" and I want to know how everyone seems to know this!  I had my anatomy US way back when and at that time baby was in the 40th percentile, but that was forever ago and did not tell me how much they think she weighs now. 

Just curious because I wish I knew how big she is!