What, what, what....explain??
I mean, we know that Keiko was somewhat boy crazy on her younger years and the idea of being mother was her dream. If you go back at the first issues of the series you will notice that she put certain distance between her and Ran, distance that has become smaller and smaller of late. As this has happened, Ran has became more bold about her feelings for Keiko so isn't that much of a stretch(heh) to assume Keiko is at least considering the possibility.