Swaddling is a case by case issue, we have learned. Our son loved and needed more swaddling. Great sensory feedback. He needed that comfort for soothing. To my understanding, the brain , the bodies muscle tone and temperment affect development more than anything else.
My daughte had to have her arms out and eventually legs,, to this day, kicks all blankets off and frankly, probably did not like the swaddle, which took us by surprise as that is what we believed in and thought we would repeat. Silly us for thinking one size fits all..They show you what they need..
Here is my biggest encouragement for milestones...enjoy them if you can and no matter what, encourage crawling as long as possible for brain development and also, read The Hurried Child..not because I think you are but because there is a lot of soft pressure on parents and children and it does more damage than people understand..skipping stages disorganizes a a young brain and their soul..hence, a growing epidemic of special needs children, IEP and a frustrated culture.