I don't think you'll be stuffing your company around by taking the 12 months and then deciding while on maternity leave whether you want to return or not.

At this stage, no one should expect you to make a decision about what you may or may not do in 12 months time. As long as you give your company at least a months notice prior to your intended return that you won't be coming back, they can't expect anything more than that. If you find another job in the meantime, you're not doing anything you wouldn't be doing even if you were still working full-time.

I would definitely be taking the 12 months if I were in your position.

love
sushee