Wife wants another baby so soon after losing many. How do I convince her not to?

My wife has had a lot of pregnancy problems, she's lost 7 babies so far, but she wants to keep trying to have a baby, and she's talking about surrogates and everything. We have two girls, three years old and four months old. But we've always wanted a big family, just not at her health expense! How do I convince her that it's a little too soon to be thinking about more babies yet?