Start applying now: Federal hiring can take a looong time, even without sequestration and hiring freezes like we have now. And, speaking from experience, interviewing while pregnant feels awkward but is not necessarily a deal-breaker for them.
That said, also know that you will not get any maternity benefits as a Federal employee. There are lots of threads about this if you're interested in the details, but bottom line is you can only count on getting the sick and annual leave you've earned. Some agencies will advance you a certain amount of time yet to be earned and some (I think probably most) will let you take at least a little unpaid leave even if you are not FMLA eligible yet, but that varies.