Spoiler warning if you haven’t read Persephone.

The question asked by a reader was “I get why Persephone didn’t think to ask, but how come Hades didn’t immediately realize Zeus was still alive by the fact that he had a sixteen year old daughter?

That’s a really good question. Gods get a lot of perks that humans don’t when it comes to reproduction. For instance, children are a consensual choice between two divine partners. So, it’s not outside of the realm of possibility that god magic allowed Demeter to postpone her pregnancy until she felt she’d charmed enough priestesses to maintain worship to keep herself and her child alive. At least that’s my theory.