Oh that's so exciting! Keep us posted. I love stuff like that lol. As asked above...did your mil know you were pregnant before she got it done?
Was the pregnancy news a surprise to your mother-in-law or did she already know you are pregnant? It would be more fun if the mother-in-law didn't know. If she did know, well, some people who read cards are just super talented at being able to tell the kind of things the folks who step in to see them might be interested in hearing, and they say them. In other words, they are great at reading subtle cues in the reaction of the person, and they pile it on.
If she (MIL) didn't know you are pregnant, then the question is, are there twins in your family? (Not hers, but from your mother's side or your father's mother's side.)
My parents always told me that that's some diabolical stuff. Don't know it might be true but just like that remember that there's a good and there's also a bad.