Whatever you do, don't fall into the "engagement ring is so many months salary" bs. I don't even have an engagement ring and guess what - it doesn't change a thing about my relationship. Well, aside from not being in debt over jewelry.

I suggest that you sit down with her and that both of you express your expectations. Ask her what she expects to happen for the rest of the pregnancy, and then when the baby is born. Is she planning to be a stay at home mom? Is she planning to make a career out of partying? Is she going to be responsible? Be explicit in your expectations for the relationship. It's acceptable that she'll need to get out once in a while, but if she is planning on making a serious habit of spending money that you've made for her own selfish reasons, that's not responsible. That's not being a mother or a fair partner to you.

You'd best both talk about this sooner rather than later.