The five-month-old fetus is about 20 weeks old, and the 4D ultrasound screening is performed at the actual gestational week of the fetus between 18 and 24 weeks of gestation, which is also known clinically as a tertiary obstetric ultrasound examination, and is the most important ultrasound examination among all prenatal fetal examinations during pregnancy. The 4D ultrasound examination is suitable for all pregnant women, especially those with fetal malformations detected or suspected during previous perinatal care, or those with high-risk factors for both the fetus and the mother. The main reason is that the period of 4D ultrasound examination is the stage when the fetus and its appendages can be most clearly displayed in the mother’s uterus, and during this period, many abnormalities of the fetus can already be manifested. The usual first-degree obstetric examination requires the ultrasonographer to keep at least 10 images, but in the 4D ultrasound, which is the third-degree obstetric ultrasound examination, it is recommended to keep at least 36 ultrasound images, which means that the items examined in the 4D ultrasound examination are more and comprehensive. 4D ultrasound with stereoscopic imaging can present the face of the fetus as well as fetal malformations more visually. Five months is the best time to have 4D ultrasound, and it is also the most important time for prenatal malformation screening during pregnancy.