Babies first explore their hands with their mouth, gradually using them to explore and reach by 3-6 months. Yet even at the stage of independent sitting, a baby can still be fascinated with those wiggly bits at the end of his arms.
Babies with an early brain or nerve injury rarely go through this early stage. They start off in the newborn period with a problem of alignment. The most common early issue is low tone or hypotonia making it hard to get the hand(s) to the mouth. Parents can help at this stage by bringing the hands to the mouth and supporting them in position. The key goal is to keep the hands in the visual field. Babies with vision problems can still “mouth” their hands and if you add a bell to a wristband, they will develop a sense of their hands in space.
If they are not supported in this early stage, they will still try to find their hand, but at the cost of “activity dependent tone”.