[YorkieRose]

It can be a guessing game alright... I try not to look closely at their belly every day...a slow change is not as noticable. At 5 weeks many have no visible signs, then as a rule, 3 to 4 days later they swell up quite large over night it seems. Many have a clear discharge at this point, others wait a longer...I treat all bred females like they are pregnant, until proven otherwise...I have had so very few to miss over the years, I just take it for granted they are...they have what I call "milk ridges" along the nipples, they are plump and run the length of the belly..much like a veggie plants in a garden..
I have found a bitch who has had pups once or more times tend to show quicker..I think it may be due to the muscles not having the same tone to hold the belly/uterus tight to the body...but that is just my theory, certainly not a science...