This is an extremely difficult question and a very sad one.
When did I first consider myself a mum, maybe when my niece turned to me for love when she fell at sports day and her mum wasn't there. Or when my dog lay with me on the floor as I lost baby after baby after baby, until I reached eight and was told no hope.
Or when I fostered babies and muddled along at the school gates in a tiny rural place, and felt part of it, included. Until they said mums night out, you will be included one day sheepandcaravan.
So I think it's a society thing as much as a personal thing.
Because when I fell pregnant again, and carried, I didn't feel like a mum, she's now three, her sister one. Every day I feel like an imposter, because so many people excluded me from that mum feeling. And for many other reasons, so I suppose for me being a mum is many, many things, each personal to the individual