How does everyone know their baby's weight?

Heather
Is it weird that I will not be able to know how much my baby weighs until he is born? (Even an estimation) I'm 27 weeks right now. My doctor says that because I'm young and the pregnancy has been going well that the only things we will be doing in the appointments from now on will be fundus measurement and the doppler. I'm just confused because it seems as though other woman my age still have at least one sono in their third trimester to check on fetal weight and postion before birth, and yes I know sonograms are not 100% accurate. Just curious.