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. But it is also not unusual for lactation to start after the pups have all been born!The most reliable way to tell when the litter is due (beside counting 9 weeks 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 to around 98F degrees.
Taking the temperature throughout the day will help you "catch" the temperature drop and time the onset of hard labor.
Here is a link to a very affordable and veterinary-school-quality book about dog breeding. This is something you can use to further prepare yourself and have on hand when the time comes:
http://www.us.elsevierhealth.com/product.jsp?isbn=9781416031390
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Retired Veterinary Technician
Lhasa,Shih Tzu Breeder/ B.A.Neurophysiology & Animal Behavior/I use plain English!