You have my sympathy...
I'm afraid there's only one thing that will stop this for her - and that's you getting married, which will mitigate against your apparent 'sin' of being together before wedlock. I should explain that although I'm not Catholic, and will disagree as to the rights and wrongs of everything I'm about to tell you about, i was raised Catholic, and though our family was not devout it's easy to see how you ended up in this situation.
What most people fail to understand is that for a devout Catholic, this is totally unacceptable, and sinful. Unmarried parents - especially women - used to be ostracised and sent away - see here:
http://en.wikipedia.org/wiki/Magdalen_Asylum
This was NOT seen as extreme - it was normality.
Your partner's mother will have been brought up, not just to pass on these values, but will have been educated, brainwashed, and possibly beaten into knowing and repeating the Cathechism by heart. And she will, with every bone in her body, believe that her son's immortal soul is at risk, as is yours and your baby's.
She is not to blame for this - it's the way she was brought up. And it's not a choice for her - it's what she holds true, and will have crept into every aspect of her schooling, home life, friendships, relationships etc. Remember, for every single aspectof life, from birth to death, and everything inbetween, there's a Catholic religious ceremony attached and this will reinforce her beliefs.