The thing about parents is that when they have kids (i.e. you), they always have expectations for how it's going to go. Unfortunately, they rarely realize what it actually entails, and that you are your own person who may make choices in life that don't result in what they want. My parents have always told me the same things to me, that I'm supposed to have grandchildren, just because they want to play with the kids. I hate kids, always have, and never want any of my own, but my parents are still always saying things like "We'll save your old toys for when you and your sister have kids." She said that exact thing to me this morning, in fact. Even though I've never in any way led them to believe I wanted kids, my mom is convinced I'll change my mind and have kids just because she expects things to work out that way.
Chances are, she'll never listen to you about it unless something happens that gives her a reality check. Just do what you do, be honest with her if you can, and let her find out what happens with your life on her own. She'll come around sooner or later.