As well, Michonne and Rick are two top-tier leaders within the communities, both from mental and physical standpoints, so it would be fairly problematic to have either or both once again dedicated to raising a newborn child.
For one, the supply situation still hasn’t fully been worked out (although there are less Saviors around to eat the food).
Rick and Michonne bringing a kid into the world is a more complicated concept than one might think, judging from all the warm feelings invoked by the episode scene.