How do I tell my mum I'm pregnant?!

I'm 24 and in a 7 year relationship with my boyfriend. I've found out today that I'm pregnant (unplanned) I'm only 12dpo (if you're meant to count from last day of ovulation, I'm not sure?) but I've done a few tests with faint lines and I'm pretty sure I got a clear positive this evening. Now if I had a while to get my head around it I wouldn't have to worry about telling my mum but she's meant to be taking me to hospital on Wednesday for an operation under general anesthetic which I may have to cancel now... so I have to tell her tomorrow 😵😵😵 help how can I tell her?!