Pregnancy extensively affects your balance and joints which is why you should be cautious while attempting jumping jacks. Also, it depends on your body type and your overall health during pregnancy to safely say that you can try this exercise. Some women do not have any issue with vigorous exercising until delivery but only with clearance from their doctor. During the 2nd and 3rd trimesters, it is important to get the okay from your health care provider to do vigorous exercises.