I'm so sorry about your mum.
This is still very early days and you will be in shock. It's not a good time to make such major decisions.
I've name changed because this is really identifying.
My mum died from breast cancer when I was a bit younger than you. It felt like my entire world had just caved in on me. I'm really sorry this is happening to your family.
I had almost the exact same thoughts as you in the first few days and weeks after we got the news that her cancer was terminal. The thought of her missing out on being a grandparent and me missing having her support was heartbreaking. Suddenly the whole vision of my future had been snatched away.
We thought we'd have about a year, maybe a little more, with my mum from the point of getting that news. She still seemed so fit and well at that point. So well I couldn't believe it was really happening. I started thinking like you, that if I got on with it quickly I could have a baby in time for her to meet it.
Unfortunately she deteriorated incredibly fast and died within 2 months of getting the news. Cancer is not predictable or reliable.
So whilst I understand where all these thoughts are coming from, please think carefully about how you would feel if - possibly worst case scenario - you managed to get pregnant easily but then she deteriorated and died while you were still pregnant. You wouldn't have time to heal or grieve before your baby arrived, and would have to deal with losing her whilst pregnant.
Somebody else suggested a memory box. My mum made one for me before she died. It has things from her life that she wanted me to know about her and who she was, things she'd saved from my childhood, the things she saved for when I had my own children, photos, and things that I chose and added. We went through it together before she died, and in some way it's the memory of creating it with her and what it meant to her that comforts me more than the contents.(I think the box we used for mine was a 50litre plastic one so it doesn't have to be small! You can put whatever you want in it.)
This is a horrible thing that you're going through, but maybe focus on the time you have with your mum. The chance to open your heart to her, ask the questions you had delayed, have the conversations you haven't had time for, spend time together doing normal things, think about how you keep her spirit/memory alive and take her with you into the future.
If you create a memory box together - or maybe one for you, and one for your future children - it will give you a chance to have some of those conversations that might have been too difficult on their own, but do become a bit easier when your attention is focused on the objects you're handling rather than intensely listening to one another with nothing to break the tension.
Making one for your future children would give you the chance for her to be comforted knowing she won't be forgotten and will be a part of their life in some way, and for you to ask her what she'd like them to know about her, what she'd wished she'd known before she became a parent, etc.
I don't know your mum obviously, but mine became extremely distressed about having to leave us all, not being there for the big moments in our lives, and the prospect of us forgetting her. Going through the process of creating a memory box and having the space to start those conversations about how she had influenced the person I was (and therefore was always going to be part of my life) and how I would remember her, brought comfort to her too. The conversations she'd been saving up to have with me over the next ten, twenty years of my life were all in condensed into a few weeks, but it was helpful for both of us to cope with the situation.
My mum also left me a letter behind. If I had my time again I would have asked her to also leave me behind some of the advice she would give me for my future.
I remember so intensely trying not to cry and be brave. Then one night I sat with my mum talking, and she cried and I listened and we talked. At the end she told me she hoped I was crying as well and not bottling it up for her. Later she sat with me with her arm around me and we just sat and cried together. We were both afraid and both hurting. It helped to be there for each other.
It is ok to cry about this, it is ok to cry with her. You need to let yourself grieve this news.
Some people might not understand that losing your mum so very, very young is not the same as losing an elderly parent, but you shouldn't box up your pain and grieve just because of them. It is you experiencing this, not them. You need to be able to express your feelings. Please don't feel guilty or ashamed.
Once again, I'm truly sorry for your news. Be as gentle with yourself as you can. Take care.