This is as much her grand child as it is your baby. Do not expect things to be all happy at first. It is however time to mend your relationship with your parents. You need that to support you and get you through the pregnancy. Tell your mom that you are sorry your relationship deteriorated but you need her support. Ask her for her support. Very few parent will refuse if asked.
Keeping it a secret does not serve any purpose, except delaying the neccisary. Write her a note, send her an email, speak to her, do whatever you have to do, but get it done.