Board Thread:The Heroes of Olympus/@comment-5896827-20160731081509/@comment-27518339-20160808235114

Perhaps Apollo changed Darren's form into that of a female, and she then became pregnant with Kayla. I would imagine that after she was born, Darren simply changed back to his original form.

(There was this one story of a girl who was to be sold as a slave by her father. She prayed to Poseidon for a disguise of some sort; hearing her prayer, the god then changed her into a man.)