Hi, my 7 week old is rolling onto one side when sleeping in a sleeping bag with arms in. When she does this she is close or touching the edge of bassinet which has mesh sides. It’s only onto one side and doesn’t roll the other way. When she is swaddled in a wrap or in her sleeping bag arms out she doesn’t roll and stays in the middle of the bed, however she does still have a big startle reflex and often wakes herself up. Should I keep her arms out in the sleeping bag? Thanks

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make her new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

How does she go with tummy time when awake?
Increase her tummy time when awake - more frequently if she is not tolerating long periods on her tummy.
Increased tummy time when awake helps to build upper body strength to assist with rolling both ways.

Some babies like to touch their face &/or suck on their fingers - if these babies are swaddled with arms in, they will often wriggle in an attempt to get arms free.
In this case we would suggest you have her arms free (also because she is rolling.)
You may also want to move her from bassinet to cot if she continues to roll. (see info linked above)

Babies need arms free & the space of a cot to roll well.