Ok, there's a very well known part of the story in the books canon where Mirri Maz Dur tells Daenerys that she is now barren, or at least that's how Dany interprets Mirri's line:
When the sun rises in the west and sets in the east. 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.
A Song of Ice and Fire: Book One - A Game of Thrones, Chapter Sixty-Eight (Daenerys IX).
Now in the show, Game of Thrones, Dany tells Jon Snow (in the episode The Dragon and the Wolf) that she can't get pregnant, to which Jon says "challenge accepted".
The problem is that I don't recall Mirri actually saying the pregnant part to Dany on the show, so Did Mirri Maz Dur tell Dany she can't have children on the show? If so, when/where?
If not, then it's time for another question.
Many of us have doubts about the legitimacy of Mirri's 'prophecy':
Are there any proofs of Mirri Maz Duur's curse for Daenerys?
Why does Daenerys trust the prophecies of those who betray her?
But that's not the point, because Dany believes them.