I sadly don't remember who wrote it or what it was called, but it was about this group who lived in this cabin in the woods and experimented on this young woman who thought she was their daughter, by giving her drugs that made her pregnancy last longer and made her bigger. They also told her that pregnancy was spontaneous, so she was just kept in the dark about everything until they told her everything while she was in labor.

http://www.hentai-foundry.com/stories/us...r-Breeding

This is it, thanks!