I took less than 6 months myself because it would have been too hard to return at the same pace if I had taken more than that amount of time off. I am at a very senior level though.
In my experience, and those of women I mentor, if you're going to take 12 months:
come back and show that nothing changed, except you can't do all the hours now. Don't feel guilty about it, just leave when you're meant to leave.
People (both men and women) will put women in the box that says "mother" if you allow them to do so so look out for this.
You need to try and get up to speed very quickly with what happened in your absence, including changing dynamics and politics. This will allow you to quickly re-establish your influence.
I think you have to be considered though, so it isn't about taking on every new opportunity to show that things have changed but being more selective and political about what you do put yourself forward to do.
You need to work very quickly to adapt to being more effective. In my case, it meant that I adopted a one touch policy on emails, made decisions a lot more quickly, I aligned myself with ambitious and smart people so that I could delegate a lot with confidence and I didn't micromanage. The hardest thing I found was that I didn't have time for general chit and chat and this was a negative as people didn't understand that in order to achieve the same with less time, I didn't have the time to "waste".
Talk about your kids but no need to be that poster girl for motherhood!