It would not be too early to do a 4D macrosomia at 21 weeks. Most of the fetus at 20~26 weeks, all organs and limbs have developed, and the amount of amniotic fluid is also very suitable for the examination to rule out malformations. When the fetus grows to 21 weeks, the 4D ultrasound can usually show the development of each organ of the fetus clearly and three-dimensionally, which can be more convenient for checking whether there is any malformation in the fetus. Because the size, position and development of each fetus are not exactly the same, so individual fetus at 21 weeks of development is not yet perfect, 4D ultrasound examination may not show clearly, generally can be another week or two time to check again, can clearly show whether the fetus has malformation.