depending on what stage of pregnancy you're at, I assume you have considered the alternatives.
you can choose to live where you like but I would think boyfriend's mother won't be overjoyed at financially supporting you long term, and she shouldn't be expected to look after your baby. you may be entitled to some benefits and need to check this out at your local Jobcentre. you can check out whether you're entitled to any housing with the local council.
but you need to grow up some. a baby is a real responsibility, and as a mother you have to ensure your child has a happy life.