"When the sun rises in the west and sets in the east," said Mirri Maz Duur. "When the seas go dry and mountains blow in the wind like leaves. When your womb quickens again, and you bear a living child. Then he will return, and not before."
This might be a real prophecy, or it may simply be a sort of curse to strike terror and despair into Daenerys's heart. The conditions put forward have a distinct ring of fairy-tale about them, and with Martin this could just as easily be taken to work against the likelihood of them coming true as in favour of it. For what it is worth, however, it seems just possible that A Dance with Dragons has satisfied most of the conditions if read metaphorically. Consider that Quentyn Martell -- "the Sun's son", referring to the Martell sun -- journeyed from the west to the east, where he died, that in Daenerys's last chapter she notes that the Dothraki sea is going dry and the grasses are dying, two of the pyramids of Meereen collapse in smoke and ash following Viserion and Rhaegel rampaging through the city and that she menstruates (or possibly suffers a miscarriage). This of course leaves us the need for a living child, and then the "return" of Drogo... though our speculation is that if she has a child, it will be a son that she names Drogo, thereby fulfilling Mirri's claim.