Mid-pregnancy fetus sudden rapid shaking for a few seconds may be intrauterine normal activities, may also be caused by hypoxia. 1. Normal intrauterine activity: the mid-pregnancy fetus suddenly trembling rapidly for a few seconds may be due to external sound stimulation, vibration stimulation and other factors caused by the normal intrauterine fetal movement phenomenon, generally shorter duration, do not need to be too nervous. 2. hypoxia: if the fetus for a short period of time for a few seconds of rapid shaking, may be increased fetal movement, is the early phenomenon of intrauterine hypoxia, this time the need to go to the hospital to monitor whether the fetal heart is normal, such as hypoxia need to be timely inhalation of oxygen to improve the situation. Fetal movement should be closely observed during pregnancy, and if abnormal fetal movement occurs, the cause should be examined and treated in a timely manner.