marriage sounds like your best option tbh...? you obviosuly want to be together, want to offer this child a secure and loving home and family unit and obviously want to keep the mother-to-be in the country... win-win, no?
PS, I'm not for one second suggesting that for you to be able to offer a secure and loving family unit to your child means that you HAVE to be married to do so, but in your case it would probably work out best to ensure the mother of your child gains citizenship and you are not separated due to immigration laws...
PS, I'm not for one second suggesting that for you to be able to offer a secure and loving family unit to your child means that you HAVE to be married to do so, but in your case it would probably work out best to ensure the mother of your child gains citizenship and you are not separated due to immigration laws...