A YA novel about a marginalized child in medieval times

I had a brief glimpse of a modern YA novel set in medieval England. The title was a possessive: "The XXXXX's Child." It's not Marjorie Eccles's "The Cuckoo's Child" (which is set in 1909). This novel, as I recall, is about a child who's mentally ill or autistic (no one had a name for it in those days) and is rejected by her family and community. She ultimately wanders away, never to be found again.

Can anyone identify this book?

It's not The Midwife's Apprentice, is it?

Some of the details fit: marginalized child, medival England, possessive title. But she wasn't mentally ill.

No, it's not "Midwife's Apprentice," but I daresay that IS one ripping good story. The title is "The something-or-other's Child." The kind of title that's next to impossible to search out.

Is it The Half Child by Kathleen Hersom? "When Sarah is born, it is her older sister Lucy who takes to her from the start. The rest of the family, goaded on by the villagers, concludes that Sarah, who is different from other children, must be a changeling child left by the fairies and might at any time be reclaimed by them. No one but Lucy grieves when one day Sarah disappears, not to return home again."

That could be it! (Although I was danged certain that there was a possessive in the title.) Thank you for the suggestion.

touchstone fairy: The Half Child by Kathleen Herson.

Thanks, all! I'll check that on my next library foray.

Did that turn out to be the book you were looking for?

I believe it is, and thank you all for clearing up that mystery! I'll have to make a trek to the main library here to get it . . . then I'll be able to confirm.