Hi mama! At 21 weeks, you're actually in a good window to start or continue light exercise, a lot of us nag-start din around that time, kasi parang nag-settle na ang pregnancy at mas may energy ka pa. Walking is super safe and honestly the best, lalo na kung hindi ka nagexercise before pregnancy, pero syempre better to check with your OB muna para sure kung okay sa iyo specifically since every pregnancy is different talaga!