Let me first say that in my opinion it is a bad idea to plan family matters (as important as having a baby) based on outside career conventions and constraints. I know this is very common in the US, but I believe it is a flawed practice. You should have your baby when you feel ready for it, and you can usually arrange the rest around it.
I strongly believe in the above as a general strategy. However, some short term tactics can help. First, having a baby will change everything in your life. doing it before you start your studies might change your determination to start such a demanding path.
Second, having a baby after your qualifying exams (usually the end of year 2), may be even a good idea, and many schools will agree for a short leave, or even be flexible enough to "take your time" in your dissertation process (watch, though, for the stipend issues). Many schools are now moving towards more family friendly policies, so that you can officially get some time off or other kinds of support.
I've known many PhD students who have successfully coped with a PhD program while raising babies, but I've also seen people that failed and withdrew.
a PhD program is very demanding. Raising a baby is even more (and more fun, as well).
Good Luck.