Of the four zoos in America that have Giant Pandas, Memphis Zoo is the only zoo that's not yet had a cub. Ya Ya the female Giant Panda there has conceived but so far has been unable to bring it to full term.
It is thought that one of the reasons could be is that she goes in to her estrus cycle in the holiday season which is too early compared to other pandas around the globe, which usually enter the cycle between March and May.