I've been thinking about your situation some more, and wanted to chime in to agree with the people who are saying to be sure to look out for yourself first, and not worry too much about "feeling bad" about various aspects of this. Check your employer's policies about any stay-on or pay back requirements. But don't put too much weight on how your boss will feel, how much work will be created by various courses of action at your disposal, etc. The fact is, you don't know yet what you're going to want to do at the end of your maternity leave. And you can't be sure to be paid fully through your leave etc. if you tell them your plans to not come back.
I know because a friend of mine who worked for a very reputable law firm told them mid-way through her maternity leave that she was accepting a job in-house with one of the firm's clients (a very common practice and actually desirable from a law firm's perspective) when her leave was done and so would not be coming back to the firm. The firm ended up only paying her for two weeks (or something minimal) beyond when she told them this, so she did not have the full paid leave she had planned, even though she had been led to believe that if she was up front with them she'd be entitled to take the full leave amount as initially planned. And I think she was on excellent terms with her superiors, but still was treated this way.
So, I guess what I'd suggest is:
-check policies about any repayment/stay on requirements (hopefully none)
-tell work you want to take a 5 month maternity leave. If asked what you want to do upon return, say you're not sure yet and don't want to get ahead of yourself as you know each baby experience is different. If they force the issue, either say you're hoping to come back full time, or you're considering whether part-time might be a good option but don't know yet.
-have baby (yay!)
-proceed with planned maternity leave; set tentative return date.
-as you get closer to your return date, if you've made a definite decision that you're not going back, and don't feel right waiting longer to tell them, then by all means, tell your boss. But be ready to have your pay and health insurance terminated as soon as they're legally able to terminate it.
-if you're still unsure, proceed with plan to return on said date and see how things go the first couple weeks. As others have said, it's entirely normal for someone returning from maternity leave to change her mind about how the plan matches up to reality. Worst case, you go back full time for a couple weeks, then apologetically quit/give notice, saying this just isn't working out for you or your family and you want to spend time as a SAHP.
In short, I don't think there's anything wrong about taking things one step at a time, so don't feel guilted into preemptively raising your plans (really, hopes) to not return.