I really Think morgana is way to Squishy to wait so late for a defensive item im not saying you have to go a full on defense built like ( http://www.mobafire.com/league-of-legends/build/undead-angel-4028 )

But something too keep her alive cause if you are playing like a support they will focus u first and take you out. And if u are going in like an nuke you still run the risk of being the first to be focused on.