I'm not Cathe but I have Diastasis Recti and when I first started the healing process, I wasn't able to do planks because I couldn't keep my core engaged. Eventually though, my core got stronger and now I can do them without any instability. So, it is unique for everyone...if you can keep your core engaged (no bulging or protrusion), then you are most-likely fine. It's best to get into a plank position and then have someone else place their hand on your tummy to check for engagement.