If there
are any actual legal issues surrounding this, I can't speak to them and you'd want to double check with a lawyer to make sure but my first personal intuition is that no, you don't have any obligation at all - other than ethical, to keep your clients apprised of what's going on with you. But, that doesn't mean you *have* to tell existing or potential clients anything at all.
We told all of our clients almost as quickly as we told our families - but both are a tight knit group with us and we were excited to share the news.
I honestly don't think the client will have any preconceived expectations of how much time you'll take off - I still haven't decided how long I'll take off completely - probably two months...I mean, we're both home and I'm hoping to get back at least a few hours a week after that time.
Now, were we in Canada we'd get to recover 55-75% of our previous year's income (assuming we paid taxes) for an entire YEAR after the new baby - imagine!

But alas, we're American...LOL
Depending on how soon you're due and how long you're planning to be out of office completely you may want to at least give your clients a heads up...what are your own personal thoughts and feelings on this?