Hi,
I think it really depends on what kind of relationship you have with your Mum. I'm really lucky to have a good relationship with my Mum - on my previous 3 pregnancies I told her pretty much straight away but I must say, I held back a week with this one because of what happened last time (I was going to wait until 8 weeks but obviously have no willpower).
I definitely think you should follow your heart on this one.