 Ultrasound has been used for over 20 years to estimate fetal weight prior to birth. Sometimes this estimation is not always similar to what the baby actually weighs. In addition to the regular measurements taken at routine ultrasound exams, 3D ultrasound of the babys arm and leg soft tissue may give us more precise information on how much the baby weighs. This measurement is called fractional limb volume. Fractional limb volume FLV is a soft tissue measurement of the arm or thigh that can be used to evaluate body composition. This measurement mostly reflects the fat and muscle development with a small amount of bone. 