Thank you for researching this really important question!
It is very common for a dog to begin lactating as early as one week before labor begins. However, many bitches do not "let down" milk until after delivery is completed and pups are available to nurse. Milk can also be a sign of False Pregnancy, since the hormones in a non-pregnant dog after a heat cycle are just about the same as during pregnancy.
Milk production is partly a matter of timing, from our point of view, and partly a matter of stimulation of the hormone system by the labor and suckling of the puppies! Oxytocin, which is the primary hormone of labor and delivery, is also the primary hormone that starts lactation.
Since there is so much variation from gal-to-gal, we just can't rely on this as a sign of impending labor.
The most reliable way to tell when the litter is due (beside counting 9 weeks, or 57-64 days, from the mating dates) is to begin taking her temperature (rectally) three times daily.
***24 hours before labor begins, the temperature will drop from her normal (usually 100-99.5F degrees in a really pregnant mom) down toward around 98F degrees.***
Taking the temperature throughout the day will help you "catch" the temperature drop and time the onset of hard labor.
***Please be sure to have your local veterinary emergency service phone numbers at hand in case you need assistance during delivery...most litters are born overnight.***
Here is a link to a very affordable and veterinary-school-quality book about dog breeding and puppy care. This is something that many breeders like to have on hand:
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