4D ultrasound during pregnancy, also known as macroscopic ultrasound, is generally recommended to be done at 22-28 weeks, preferably at 24-26 weeks, or at 20-24 weeks in the case of twin pregnancies or multiple pregnancies. 4D ultrasound is a multi-directional and multi-angle observation of the fetus’ organs, face and heart and other structures for malformations under ultrasound. The fetal structures have developed between 24-26 weeks of gestation, and the size of the fetus and the amount of amniotic fluid are moderate, so there is more space to move freely in the uterus, which is conducive to observation. As the gestational week grows, the fetal limbs will be obscured and the fetal position will be affected, so individual organs such as the face and heart may not be clearly scanned and may be misdiagnosed or missed. Therefore, the best gestational week is recommended to be 24-26 weeks. In twin pregnancies and multiple pregnancies, the fetuses are more likely to be obscured from each other and are usually done earlier than in singleton pregnancies.