Fetal legs stay motionless during 4D ultrasound may be due to the fact that the fetus is in sleep time, the fetus has a quieter character and other reasons. 1. The baby is in sleep time: the biological clock of the fetus is not necessarily in synchronization with the adults, if the time to do 4D ultrasound happens to be the baby’s sleep time, then there will be a situation that the legs have not been moving during the examination. 2. Fetal character is quieter: If the fetus character is quieter, the whole pregnancy is not too active, fetal movement is not much, the fetal heart is normal during the examination, will also lead to do 4 d ultrasound when the fetal legs have been immobile situation. If the fetal legs have not been moving during the 4D ultrasound, the pregnant woman can increase the fetal movement through appropriate exercise, caressing the abdomen, etc. If the fetal legs are still not moving, there may be abnormal fetal development and other reasons, it is recommended to consult a doctor in a timely manner, under the guidance of the doctor, improve the relevant examination, to clarify the cause of the disease, and symptomatic treatment.