"If she's going to be that way, I'd suggest just putting him in nursery all 3 days and telling her that she won't be seeing either of you until she loses the attitude."
Please don't take this advice above from another poster. Your mum is totally caught up in her grandson and appears to have lost all perspective but presumably you want to maintain a positive relationship and are not averse to her seeing you both regularly per se.
However your own life, your precious time with your son whilst on Maternity Leave should not be lost because of your Mum
I wonder if you find it hard to stand up for yourself with your Mum? Is she used to getting her own way in your relationship? Now you have a son, your Mum probably thinks that things can carry on as previously, maybe with you "giving in" with her, particularly if she uses additional ways to put pressure on such as the Nursery/aggressive story or the "all that's left for the rest of her life" comment. Problem is, your son and what you feel as a parent is your absolute priority now, as is absolutely right, and your Mum has to get her head around that.
Could you tell her honestly how you are feeling and what you are going to do? As in
"Mum, I know you love seeing ……. but 4 or 5 full days a week is just too much because I need to spend quality time with him alone/ it is stopping me meeting and getting to know other parents/I want to take him to ....etc. "
Your Mum seems likely to refer to her own needs in her response...about needing to see him, having nothing else etc etc
Your response might be some thing like
"I understand that you feel like that and find it hard.
However, 4 or 5 full days a week is just too much because I need to spend quality time with him alone/ it is stopping me meeting and getting to know other parents. Not doing these things means that I am not building up a network of friends with similar aged children, who might be ...playmates in the future. Whilst I am on Maternity Leave it is very important to me to do that. So I have booked into ….on Tuesdays and Thursdays and you cannot visit on those days"
Keep repeating a version of that whilst acknowledging that she feels like she feels.
Re the Nursery issue maybe something like:
" I know you are worried about ...going to Nursery and that the rest of the family feel the same. You mentioned research that shows Nurseries make children aggressive. I have seen lots of evidence that Nurseries help children with socialising/social skills/lost of basic skills ...etc. I want ...to experience that and I am going to send ...to Nursery for 2 days a week."
Your Mum seems likely to repeat her concerns. Maybe she will say that she doesn't understand why she can't look after him for 3 days. You could reply
"I have seen lots of evidence that Nurseries help children with socialising/social skills/lost of basic skills ...etc. I want ...to experience that and I am going to send ...to Nursery for 2 days a week. Do you want to look after him one day a week or not? If you don't then I will book him in to Nursery for 3 days a week!"
"I would love you to look after him for 1 day a week and I know he will have a fun time with you, and you two can build a close relationship (like I had with my grandmother???) and I want him to go to Nursery for 2 days a week so that he has different experiences and builds other skills and relationships as well. Do you want to look after him one day a week or not? If you don't then I will book him in to Nursery for 3 days a week!"
In this type of conversation it is best to decide categorically what your decision is, tell the other person, continuously acknowledge what they tell you they are feeling etc but then keep repeating what your decision is ...and whatever you do want them to do, keep asking if they want to do it or not.
Hope this helps. I think it is particularly hard when it is with a parent and inevitably one is in patterns of relationship behaviours built up over many years from childhood. 