Congratulations - tell her NOW!
The only reason for hanging fire till the scan is to take account of the remote possibility that the pregnancy might not stick - and if, by some sad mischance, it does not, then Mum will be there by your side to comfort you, having shared the initial excitement.
I am sure that all will be fine.
I knew when all my DDs were pregnant - except one of them, who I had not expected to have children, so that was a bit of a shocker - but lovely!