In season one, Daenerys was cursed by the healer and maegi Mirri Maz Duur after Khal Drogo sacked the woman's Lhazareen village. Duur used her skills to "heal" Drogo, actually introducing an infection that left him near death. Duur later used her blood magic to kill Dany's unborn child, leaving Drogo healed but in a vegetative state. Daenerys asked when Drogo will be as he was, and Duur replied maliciously, "When the sun rises in the west, sets in the east, when the seas go dry and the mountains blow in the wind like leaves." On the show, her curse ends there, but in the books, it continues: "When your womb quickens again, and you bear a living child. Then he will return, and not before." In other words, never.
Although the line about her womb was never delivered to Dany on the show, she's been operating under the impression that she was barren ever since. During season seven, her childbearing abilities come up several times, and Dany tells Jon herself more than once that she can't have children. As he points out, the witch who killed her husband might not be a reliable source of information. With this much focus on the topic, and Jon and Dany finally giving in to their attraction in the season seven finale, it wouldn't be surprising to see Daenerys become pregnant in season eight. This could be bad news for Daenerys if Jon is Azor Ahai. According to legend, Azor Ahai had to sacrifice his wife Nissa Nissa in order to forge the sword Lightbringer, which could possibly foreshadow Daenerys dying in childbirth.