WHEN: Most babies are ready to transistion out of their swaddle and go into an arm's free sleeping bag between 16 - 20 weeks of age. If they are consistently rolling from back to tummy earlier than this, they need to sleep unswaddled straight away (for their safety), and popped into a sleeping bag instead, where they have their hands and arms free (to allow them the freedom to lift their head up to reposition themselves).
HOW: There are a couple of different ways you can transition to a sleeping bag. You could start by unswaddling one arm for the morning nap, and/or one arm when they first go to bed in the evening when sleep pressure is high / melatonin is on the rise (re-swaddling this arm after they wake for a feed for the first few nights if you like). Once they are happy to sleep like this, progress to one arm out for all naps and night. Some babies don’t respond well do having just one arm out, and adjust much quicker (in 2 – 3 days) after having both arms out in one go. It’s normal and we would expect additional fussing to begin with as they adjust to being “arms free”. If a baby is really struggling to come out of a swaddle, an arm's up swaddle (i.e. LTD) can work as a transitional swaddle for a few weeks.
By 5 - 6 months old, a swaddle becomes an unhelpful sleep association, as they want and need the freedom to move around and get comfy in their sleep, plus are really quite strong by that age and are likely to break out of it - leading to more night wakes, as they'd rely on being re-swaddled to get back to sleep. In contrast, the sleeping bag becomes a helpful cue that it's bedtime, helps regulate body temperature and means they won't need any additional blankets in their bed.