Your doctor does not ask you to lie down and then press on your belly? This is measuring your fundus or the top of your uterus. My doctor never told me my measurements until he found that I was measuring small...He also never told me my baby's heart rate until I asked...He should ve started measuring you as soon as you became pregnant. This happens by internal exam. He puts his fingers inside you and presses on your lower belly to measure your uterus in early pregnancy. At this point your fundus should be well above your pelvic bone.
Do you mean with a tape measure? This can be done as soon as the uterus starts to rise out the pelvic cavity, around 12 weeks of pregnancy. The measurement in cm should roughly equal the amount of weeks along you are. Ask the doc to do it!
What does it mean when your doctor says your measuring early for my due date and the baby is sitting to low?