User blog comment:FairyRaven/Fairy Tail Chapter 453: A Father's Job, Review and Commentary/@comment-24277053-20150920140428/@comment-7418318-20150920141356

Levy and Gajeel spent quite some time together, maybe even up to a full year.

That was plenty of time to get Levy pregnant the "classical" way without any magic.

Not that I think it actually is the case, but it is certainly not impossible.