I could lactate before I was pregnant. Not sure why, as all my hormone levels etc were normal, I put it down to my wanting a baby so much and working with babies and mummies every day.
I then got pregnant and stopped!!! for the first half of my pregnancy I never got a drop.
it started coming back about 22 weeks I think... was between 21 and 25 weeks. and has slowly been increasing, from just a drop or two when I actually express, to the day before yesterday I was in the bath just relaxing and looked down and noticed I was leaking =)