An ultrasound predicts the age of a pregnancy according to fetal measurements (diameter of the head, abdomen , and length of the femur) compared with standard statistic values. The predictions are always approximate (not 100% sure) and do not indicate something wrong with the fetus. So, if you child measures at 18 weeks, this means that the fetus is about the size of an average fetus at 18 weeks (even though you are 21 weeks pregnant). To promote fetal health, you can begin t be more conscious of your diet. You can eat foods that are rich in nutrients. Vitamin and mineral supplements are also recommended in order to promote your baby's health.