If you're 110% against it, this is typically a dealbreaker and it will be easier to just flush it out now, rather than creating angst and tension in the relationship. It's not what you originally agreed to, and it may be worth counseling to see how flexible you both are.

If you're not 110% set against it....

When kids want a puppy, typically parents will test their sense of responsibility with extra chores. Do the same thing. Let's be honest- she may have too much extra time and energy, whereas because you're doing everything you don't and that's not fair. Lay it out as "I take care of 1, 2, 3, 4, 5 things to keep our lives running smoothly. Frankly it's too much, as the only free time I get is on the weekends. I think that having a child is just too many things for me to handle the majority of on my own. If we can come up with a compromise where you take on more of the household chores, then I'll have more time and energy to reconsider in a few weeks/months." This compromise could include her taking another job that's closer to home, doing the cleaning (or hiring a cleaner- we pay $65/week and it's great because all we have to do is trash and dishes, wiping up major messes during the week), and looking after her own car, or whatever means a more fair division of labor. Discuss options for things that could go wrong (we are about to have this talk as I have zero problems aborting a down-syndrome/deformed fetus, because while I'm willing to have kids for his sake, I'm not on board with a kid that will need me its whole life), and just how badly she wants kids-is it something you are willing to try, but not invest tens of thousands in fertility treatments if you can't conceive naturally? There are tons of other details to sort, but a fair division of labor and a reassessment of both your thoughts are a good place to start.