A number of fetal measurements are taken by ultrasound and placed in a formula to create an estimated fetal weight (EFW). The main fetal measurements taken for a growth scan include: head, abdomen, femur, and the humerus. The EFW can be used to help determine whether the fetus is average, larger or smaller in size for its gestational age. These measurements are most precise early in pregnancy with up to 15% error later in gestation due to difficulties with late pregnancy imaging and acoustic shadowing. If a fetus is at less than the 10th percentile for its gestational age, your physician may send you to a MFM for further fetal assessment and wellbeing testing.