4D refers to 4D ultrasound, which is generally recommended between 20 and 26 weeks of pregnancy. In general during pregnancy to do four-dimensional ultrasound, generally recommended between 20 to 26 weeks. This is because the fetal organs have matured during this period, and the amniotic fluid level is more appropriate at this time, with good fetal mobility to facilitate judgment. 4D ultrasound is able to observe the growth and development of the fetus through 4 dimensions in a direct and three-dimensional way. 4D ultrasound can more accurately determine whether the fetus has congenital malformations, such as congenital cleft lip and palate, fetal hypoplasia, congenital spina bifida, and heart disease, which is conducive to the timely detection of the problem and timely treatment. As it takes some time for the embryo to develop into a fetus, it is not easy to see whether there are malformations in the fetus when you do 4D ultrasound before 20 weeks, as the fetus is small in gestational age and the subcutaneous fat of the fetus is very little, and it is not easy to see whether there are malformations in the fetus when you check the 4D ultrasound too early. And more than 26 weeks check when the amniotic fluid is relatively reduced, and the fetus is relatively large, poor mobility, can not clearly observe the fetal development. Four-dimensional ultrasound examination belongs to the large screening examination, after pregnancy needs to be closely good pregnancy test. Choose a suitable time to do 4D ultrasound examination, in order to clarify whether the development of the fetus is normal.