There are a few sleeping bags special for newborns but most of the 0-6 m ones have a weight requirement.
Newborns they don't move much and they also get hot or cold very fast, sweat , vomit etc...
Layers are a better idea for the first weeks as you can easily add and remove without wake them.
When she was tiny what I did was to put a burp cloth/muslin/a small sheet across where the head would be so I can fast drag it under her if she brings up her milk, then when I was feeding her I had her loose wrapped up in a blanket so if she was falling asleep eating I could just put her down without her waking from the difference of temperature when she touches the cold Moses basket.
After that I would add more if needed or just unwrap a little the blanket if I thought she was hot.
Amelie went in her sleeping bags at 2 m old when I stopped doing a night change of diapers.she was out of the size 0-6 at 3.5 m(long baby) so don't invest in too many of them.
Now she is in 6-18 m ones and we will soon have to find sth bigger as at 9 m she almost outgrew those too,