Origins of "up the duff"

All the slang sites list the phrase. I saw one or two mention that the word "duff" means a boiled or steamed pudding often containing dried fruit.
Somehow they connected the pudding connotation with the phrase "bun in the oven" ( another phrase for being pregnant ). I dont get the connection so maybe someone else can explain the duff, pudding and "bun in the oven" relationship.