User blog comment:Miskos3/Fairy Tail 525: Why Did the Emperor Never Love His Child?, Review/@comment-5303512-20170304145630/@comment-4829046-20170305003155

All interesting point and all but....

I think we should all just chalk it up as

"Ankhseram logic"

Add in the part about love and magic, and you could associate it with the curse somehow doing some fuckery to imbue life into Mavis' womb. Perhaps maybe her ovaries had ceased function due to the body going immortal and thus having no reason to reproduce.(I'm just guessing there, so don't quote me) Or, maybe the curse would generally kill any embryo before it started to form and the only way to contradict that was to exchange the life of the mother or something.

Zeref not loving the boy may be explained as being linked to him resenting the curse for doing that.