Thanks everyone for your thoughts. Its great to have people be honest about something that is really close to their hearts so thanks heaps for sharing, your collective wisdom has enabled me to avoid many possible mistakes.
My choices are to not invite her to the scans, and I will treat her as I always do. I won't tip toe around her but I will also try to be sensitive and keep discussing with her the journey she and her DH are on at the moment like I have been for the past 12 months.

Thanks heaps, your comments and thoughts are muchly appreciated.

Joy.