Transitioning from swaddle to sleeping bag

[Cosmo89]

DS is a small 16 weeks old and has been put to sleep in his swaddle for 8 weeks. The only way we can do it is to let him fall asleep on us (shh patting) and gently putting him in the cot when he's asleep/nearly asleep.

I've tried putting him in the cot when he's not in the swaddle and has fallen asleep but he's always woken himself up with startled arms as he's being lowered down and not been able to resettle himself after.

He's now getting too big for the swaddle and we need to move on.

I'm terrified! We've only just got him in a relatively good (well, better) place sleep wise and I really don't want to go back to no sleep etc as I'm still knackered

Any ideas? Help!!!!

My DD moved from swaddle to sleeping bag about the same age but we started the transition from about 11 weeks with just hand poking out, then one arm, then both arms. She would usually escape before I "officially" moved her amount of swaddle-ness though. Are you using blanket/sheet or the wraps? Just as if you want to start weaning her from it now you can get bigger swaddle wraps from swaddleme and start doing one arm then both for a few weeks.
I find if she has fallen asleep on me/in my arms that if I keep her right against me until she's only just above the mattress, so bent right over the cot with her still held close, then only moving her away at the very last minute, the arms stay pretty still.

HTH and good luck!