Before 12 weeks of the pregnancy the uterus is contained within the bones of the woman's pelvis and cannot be felt through her belly. After about 12 weeks, the uterus begins to grow up and out of the pelvis and is able to be palpated by your caregiver (and you), usually by just being able to touch the very top of the uterus (or the 'fundus').
Some women with subsequent pregnancies will have a uterus that is a little more 'bulky' from previous pregnancies, making it move up out of the pelvis earlier (possibly after about 10 or 11 weeks). This may also be the case if you are having twins or more. If you feel you are 12 weeks pregnant or more and your uterus cannot be felt by your caregiver, it may be that you are not as far pregnant as you first calculated.