The preview was edited heavily as usual so chances are Meredith wasn't even saying that to Izzie, if she was I kind of don't expect the pregnancy to get that fall along. Its probably just an Izzie will lose the baby contrived drama storyline if anything.

Does Shonda think its actually realistic for none of the couples to ever have children? If she plans for the show to go on for any extended period of time someone's going to have to have a baby. These people aren't getting any younger and it would be unrealistic for none of them to want children and we already know Derek and Meredith do.