Part your normal prenatal checkup after about 20 weeks includes your practitioner doing something called a "fundal height measurement." He or she takes out what looks like a little tape measure and actually measures the distance between your pubic bone and the top of your uterus (called the fundus, hence the name "fundal height"). This is a quick way of measuring the baby to make sure that his or her growth is on track.
In general, the number of weeks you are should equal the number of centimeters your practitioner measures. So, if you are 30 weeks, your fundal height measurement should be somewhere between 29 and 31 centimeters. I often call this measurement a "poor man's ultrasound." We all use it in the office as a quick way to assess fetal growth.
If a measurement is bigger, or like in your case, smaller than expected, an ultrasound in warranted. If your doctor said that your belly was measuring too small, he or she should have recommended an ultrasound to more accurately assess the size of your baby. If you haven't been offered one, you should talk to your doctor.