The 二口女 (Futakuchi-Onna=two-mouthed woman) is typical of a few other female yōkai (such as rokurokubi, kuchisake-onna and the yama-uba,) women afflicted with a curse or supernatural disease that transforms them into yōkai. The supernatural nature of the women in these stories is usually concealed until the last minute, when the true self is revealed.
(More on them later this week.)
A futakuchi-onna is characterized by her two mouths - a normal one located on her face and second one on the back of the head beneath the hair. There, the woman's skull splits apart forming lips, teeth and a tongue, creating an entirely functional second mouth.
Though there are several stories linking the appearance of a futakuni-onna's second mouth to different causes, it is most often linked to how little a woman eats. The soon-to-be futakuchi-onna is usually a wife of a miser and rarely eats. To counteract this, a second mouth mysteriously appears on the back of the woman's head. The second mouth often mumbles spiteful and threatening things to the woman and demands food. If it is not fed, it can screech obscenely and cause the woman tremendous pain. Eventually the woman's hair begins to move like a pair of serpents, allowing the mouth to help itself to the woman's meals.
While no food passes through her normal lips, the mouth in the back of her head consumes twice what the other one would. In another story, the extra mouth is formed when a stingy woman is accidentally hit in the head by her husband's axe while he is chopping wood, and the wound never heals. Other stories have the woman as a mother who lets her stepchild die of starvation while keeping her own offspring well fed; presumably, the spirit of the neglected child lodges itself in the stepmother's body to exact revenge.
Maybe she'd enjoy some pancakes...