My nephew was very slow to reach his milestones and needed a couple of visits from a physio to show his mum the sorts of activities that he needed to develop his muscles. So some babies need to be encouraged to sit or roll over etc while others will manage this on their own .

It is important that children experience crawling to help develop pathways in their brains. Some children experience problems with not being able to cross their midline when they are older which can lead to learning difficulties that require OT intervention.