You can start leaking colostrum after the first trimester. Colostrum is thick yellow and sticky. I leaked a lot starting at 14 weeks with my daughter. My mature milk came in almost 5 days after giving birth. You'll know when the milk is there because you'll be engorged. Milk is not produced during pregnancy.
I read colustrum comes in as early as 20 weeks and your real milk about three days after delivery.