4D can be done at 20-28 weeks. It would be better to do it at 24-26 weeks for a single pregnancy and 23-25 weeks for twins. Four-dimensional ultrasound is based on three-dimensional color ultrasound imaging, plus time, can display dynamic three-dimensional images, to determine whether the fetus has cardiac anomalies, fetal anomalies, to determine whether the fetus has a broken lip cleft, spina bifida, cardiac transposition of the great arteries, ventricular septal defect and other diseases. Pregnancy 20-28 weeks can do four-dimensional ultrasound, pregnant single fetus in 24-26 weeks to do, pregnant twins in 23-25 weeks to do will be better, to do four-dimensional ultrasound is also time requirements, the examination time should not be too early, not too late. If the four-dimensional ultrasound examination time is earlier than 20 weeks, the fetal limbs and organs are not yet fully developed, and it is not possible to judge well whether the fetus has malformation problems. Check after 28 weeks of pregnancy the fetus is larger, the amount of amniotic fluid and space in the uterus is smaller, the impact of bone echoes is greater, which will reduce the accuracy of the examination. Checking for abnormalities after 28 weeks is more damaging to the pregnant woman’s body. After pregnancy, you should do a good job in all time periods of pregnancy and obstetric examination, there are abnormalities can be dealt with in a timely manner.