Do babies only sits up from tummy position when they start crawling? If not when will they start doing that? In addition, will propping up a cushion below the tummy helps them to crawl ?

Babies just love to move. We know this by watching babies who are frustrated by their immobility! Thinking they can assist children to be more mobile, many parents prop their babies up into a sitting position at a very early age. Yet, despite the best of intentions, this position can actually slow a baby’s pathway to mobility. Instead of making their muscles stronger and therefore able to propel them along, sitting actually has the opposite effect – it prevents the muscles needed for movement to be strengthened. So please don't prop a pillow under baby to help him crawl.