For those asking why I haven't told her...we have never ever talked about stuff like that, conversations about sex, bodily functions, romantic relationships were all kind of, banned, I guess growing up. She was very religious, less so now. My lesson on contraception as a teen basically went "if you get pregnant I will force you to have the baby, if I ever find out you've had a termination behind my back I'll disown you".
I would literally have to start a conversation with 'hey mum, would you like to know why I haven't had kids?'. And like a PP said above, some of those reasons would be hurtful to her, if I was going to be completely honest about it.
I wasn't asking if it was odd she hadn't asked, I was asking if it's odd she didn't know, regardless of how the conversation might have come about. It sounds like it's not odd at all, so you've all answered my question, thank you! I think because I've never had that kind of open relationship with her, and I don't have DCs myself, I have no gauge for what is 'normal' hence asking the question.
And the whole 'glad you don't have kids so you can look after me' is a whole other thread in itself! My mum isn't a particularly selfish person, but she has some funny ideas about what children 'owe' their parents based on her own upbringing, and the 'honour thy mother and father' type stuff in the Bible.