An ultrasound for fetal growth is a limited ultrasound. Although the baby’s anatomy is evaluated, it is not the main focus. There are several measurements that are obtained for the baby’s head, abdomen, arms, and legs which are then used to calculate an estimated weight. The further along in pregnancy, the less accurate the estimate of the fetal weight by ultrasound. In fact, a third trimester ultrasound may be in error by over one pound (plus or minus). Regardless, ultrasound is the most accurate way to determine an estimate of the baby’s weight.