Em - As someone who is obsessive about their career and had very high expectations about beginning my phd as soon as I had popped, I have to say that I agree with the other girls. I am going to start it next month (he will be 4 months) and am even a bit worried about that time-wise. The first six weeks really are a challenge as you learn your little bub, learn to breastfeed, try to sleep etc. And once you have them, you realise a week is a looooong time. Not only for you but for them. They are still learning who you are and that they can rely on you.
Good luck with what you decide. And it is great they will let you decide after the birth.