Tummy time isn’t a substitute for sitting up and sitting up isn’t a substitute for tummy time. This is because the two activities work different groups of muscles. Tummy time is one of the only ways your baby can work on strengthening the muscles they’ll need to sit up and reach other key developmental milestones. So, when you’re setting up routines with your newborn make sure that you are not counting time sitting up towards your tummy time goals.