I haven't read the other replies, but I'd be hesitant to try castor oil. I did it, and I don't recommend it.
I was 3.5cm, 80% effaced and had been for weeks, MP had been out for weeks. I was desperate to get my very large baby out, so I decided to try it, with the midwife's blessing. She recommended pouring 1 4oz bottle of castor oil into a blender and blending it with an equal amount of orange juice to make it easier to drink. She said to do it first thing in the morning so I wouldn't be up all night with diarrhea.
I took it around 8 am, and within two hours had the worst diarrhea ever. After a while just liquid was coming out of me, and my rear end was raw. I was using baby wipes to wipe myself each time I used the bathroom, and even that hurt. It took hours for the castor oil to work its way through me, and I had nasty oil-tasting burps the whole time too. I was absolutely miserable.
That said... I went into labor at some point that night while I was sleeping. I woke up at 2 am with intense contractions, and my water broke a few minutes later. DS was born just before 5 am, so less than 24 hours after taking the castor oil. Did the castor oil cause labor? I have no clue. But, as my midwife said, "Well, you were all cleaned out and didn't poop in the tub..."
