I need some advice girls.

My mum is having some scan results back on Friday where we find out if her cancer treatment is working or not. 

Now my dilemma is- do I tell her the night before/morning of? To get her spirits up and positive thinking? Or do I wait for the outcome of the scan the tell her afterwards? I was going to wait until Christmas but I feel from a selfish point of view I REALLY want to tell mum. But I just don't know what's best. If the scan results arnt looking good will that make her feel worse knowing she's just getting more ill and has a grandchild on the way? Or will it give her some fight back? What would you do? I've not told any one else so I can't even ask friends advice. :(