Someone used the search term “how soon before labor begin lactating”. I’m afraid she may have been looking for a way to tell when she was going to go into labor — that’s pretty typical when you get “great with child” to be on the lookout for some sign — any sign! — that you will soon be having your baby out of your body and into your arms. (While there are some indications, there is no fool-proof sign, unfortunately.)
Your body begins the preparation process for breastfeeding, actually as part of your normal menstrual cycle, but it doesn’t really “kick in” until you get pregnant — that’s why one of the earliest signs of pregnancy is more-sensitive nipples and larger breasts. (The areola — the brown portion around the nipples — also gets darker brown when you’re pregnant, and this starts early in pregnancy too.) There may be a “range of normal” for when you start producing the first colostrum (the earliest milk, packed with calories and antibodies), but it varies even different pregnancies for the same woman. When I was pregnant the first time, I noticed that I could express some colostrum a few weeks before I gave birth; the second time, it was a few months prior. It may be that I just noticed it earlier the second time; although it’s equally possible that since it was my second time, that my body started making it earlier. Both my babies were born on the exact same day of gestation, so this cannot possibly be a “marker” for when you’ll go into labor.
This is actually pretty cool, because if your baby should happen to be born premature, then you can still give him or her breastmilk — and that milk is specially designed for premature infants — it actually differs in make-up from the breastmilk of a full-term infant. While your infant may be too premature to suckle, if s/he can take any nourishment by mouth (as opposed to requiring IVs), then you can pump the milk and feed it to the baby.