That question was much, much discussed when the song came out. Some said a flower. Some said a ring. And some said a baby. I certainly didn’t think anyone could carry a baby to term in a town like that one (pass the biscuits, please) without someone noticing. And even if someone could, it hardly seems likely that the observant sibling would have missed that detail.
I’ve always believed that Bobbie Gentry actually had nothing specific in mind and left it like that just to be mysterious.