At 17 I got pregnant to an older man I was desperate to keep despite him being awful to me. I had a miscarriage and was devastated. No one could have told me I was doing the wrong thing. If he hadn't have been so abusive the night I had the miscarriage, I may have even tried again,
Now I am eternally grateful every day that little life came and went in the way it did. Not only did it mean I split with said horror of an older man, but I went on to have a million wonderful experiences because I packed up immediately after and got as far away from him as I could at the time.
I went to University, travelling, built a career here and abroad, met DH at a time we were both financially and emotionally ready to commit to having a family. I feel there is real equality that comes from of not being financially or emotionally dependent on him. I choose him because he makes my day to day life better. I wouldn't stay if he didn't.
Now with two lovely babies and a security I didn't have at all in childhood, I think back to that 17 year old girl and wonder why she was willing to settle for so little when life can be great.
OP, your mum wants the best for you because she loves you. Having this baby will be tough and you will miss out on so much. This guy probably won't be around five years in the future. Be kind to your mum and kind to yourself.
Your Mam is actually the same age now as I had 2nd baby. She knows she will end up having to help and given she will have sacrificed so much to have you, waiting until you are entirely independent is only fair.