Fresh pineapple
There is an enzyme in pineapple which is also reputed to be a natural inducer of labour – however the enzyme is only found in fresh pineapple (the process of juicing or tinning the pineapple destroys it) and is only present in small quantities. Allegedly you would have to eat as many as seven pineapples to consume enough to have an effect.
I don't think I could eat 7 in one go!