First of all, congrats. My wife and I are Prego's with our first too. The first tests they will do are blood work and a kind of sonogram to tell how far developed the baby is. By judging the development, the ob can determine more accurately how far along you are. they will measure your tummy size as you go along... consider the first measurement as "ground zero" so that when they measure the belly from there on, they can determine how much the baby is growing, the placenta, etc. and how much is your own body fat too. They measure completely around your mid section, around you back (as if you were being fitted for a very high waited pair of pants, then they measure from just below your sternum (the hard center of your chest) to just above your pubic bone. But it will be the sonograms and blood tests that nail down your progress.