Tracey
I think going to them with the offer of 6-12 would be good, it will give you time to be home with DH, it also gives you a bit of lag time if there is traffic or something comes up that makes DH late home, even by 5 mins could throw your 5pm start off!

Yah for being able to hold the power in these negotiations.

Good luck with it all & enjoy your time at home with your two gorgeous girls and enjoy your pregnancy!
I think a 6-12 shift sounds good, as long as you are not too tired from allready doing a 1am to 5pm shift with the kids.