my baby hands and legs shake when he try to grab something and when he go upstair from one step to another

A. shivering in newborn can be due to startle reflex and less likely due to calcium deficiencdy. kindly continue to give breastfeeding to the child and take a video of the shivering and meet your doctor who will tell you whether it is normdal startle reflex or it is due to hypocalcemia.