At Futaba Channel they mention that this picture is based on this story http://yukkuri-futaba.sakura.ne.jp/kariloader/src/anko041.txt. I got a clue of the plot with Excite Japan translation service, which is not too accurate: it's about a Reimu who gets pregnant of a lot of kids, who start to eat each other because of starvation and by the end just a few get born. It would be great if this short story is translated.