The current wheel you are using is fine for the next few weeks but I wouldn't wait until her back arches, I would upgrade the wheel to 12" once you notice your hammie growing.
I also know this sounds really picky but when you upgrade the wheel go to a plastic one as wooden wheels may be chewed on and splinter and some hammies use it as a toilet so it may get smelly and quite disgusting but remember when upgrading get a wheel with a solid base please
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Gimley, Gloin, Bart, Meeko, Gingerbread and Red Eyed Pig Pig you will always have a place in my heart and life. <3
Nutmeg, I shall miss you forever but I shall know you are with me always <3
Proud owner of Flo the Westie x Scottie