I somewhat get this. My brother and I were the only two for my parents. We were pretty close. When he was killed, he was unmarried with no children. I was also unmarried with no children. I had my child in my late thirties, and he is her only grand. She doesn't pressure me at all, because she's not like that but also because it would get her nowhere with me. That's just my personality and our dynamic.
The urge that I feel to have another is all about allowing my son to grow up with siblings and to give my mother multiple grandchildren. I am the only one who can do that, so there is a bit of responsibility on my shoulders. I do feel an obligation to take that into my consideration. While it is MY family and my husband and I are ultimately responsible for raising and nurturing our offspring, I do believe that--as her only living child--I owe her a place in my considerations.
I don't think that your mother means to be disrespectful or hurtful or annoying. I think that she is playfully nudging you, and it's just getting on your nerves for the reasons that you stated. Is this how you two communicate historically? If so, then she is just keeping with that, so don't hold it against her.
Ultimately, how and when we create our own families is up to us, and we should never base it solely on what others want. That just wouldn't make good sense at all. When it comes to our parents, though, there's something about that direct lineage that gives their sincere take on it a bit more weight than what anyone else might have to say. It is also THEIR bloodline that is being continued, and they have some deep-set feelings about that.
Look at your reasons against having another, and see where some of that might be turned around. Don't change your mind right away; just allow for some wiggle room and don't let certain inconveniences be deal-breakers. If it's an issue of childcare, for example, maybe your mother can work her schedule to be available to stay with #1 while you tend to #2. Or vice versa. I know that it's not that simple. My point is just to factor her into your decision. If she lives far away, don't assume that she would not be available to help and base part of your decision on that. Actually discuss that with her, if it's something that would make a difference for you. If you don't mind having her stay with you for months at a time, present that to her and ask if she'd be willing to do that to help you out. That gives her both insight to your decision-making process and a bit of control. It includes her, which I do not believe is unreasonable, especially under these circumstances.
If after all of this, you and your husband settle on an emphatic NO, then tell her that. As long as you are on the fence about it, she is going to want to influence you. It can be annoying, but at least you know where she stands.
PS. When my husband and I got together, he aleady had two children and didn't want any more. He wanted me, though, and he knew that it was important to me to give my mother at least one grandchild. He understood that I was the only one who could help my mother to realize her dream of becoming a grandmother, so he reopened the factory.