User blog comment:Miskos3/Fairy Tail 525: Why Did the Emperor Never Love His Child?, Review/@comment-5303512-20170304145630/@comment-7418318-20170304185454

Sure it would, but then we also saw her "die" from their first kiss... or at least what we assumed to be their first kiss. And I really can't see them running into each other in the forest, fuck, then get dressed again and only THEN have a nice romantic kiss which kills Mavis.

Though I guess it is possible that they had sex much sooner, back when Zeref was teaching them magic, but both survived that due to it being just for it's own sake without any love involved. Of course something or someone would have had to slow or stop the growth of the fetus inside of Mavis' belly since that happened years prior to the death kiss.