The phenomenon of frequent fetal movements at 21 weeks of pregnancy is considered to be a sign of intrauterine hypoxia. Under normal circumstances, the fetal movement should be between 4-6 times per hour, if it is lower than 4 times or higher than 6 times, it is a sign of abnormal fetal movement and needs fetal heart monitoring. If the fetal heartbeat is too fast or too slow and not within the normal range, it can be identified as intrauterine hypoxia, so you should go to the hospital as soon as possible to improve this phenomenon, to avoid intrauterine asphyxia and fetal death due to fetal hypoxia. Normally, you need to take rest in the left side position to reduce the compression of the abdominal aorta by the uterus, which causes the phenomenon of reduced blood flow.