
Often bitches will pant and act uncomfortable at the time they experience the abdominal shift (where the uterus has to fold and the belly drops noticeably), this happens with a larger litter usually 6+.
It can be confused with first stage labour by the inexperienced.
Some bitches can have a long first stage. After 20 years of breeding my current Mum was in obvious labour for 38 hours which consisted of frequent visits outdoors to poop, panting whining and even howling!!!
She finally whelped day 64 from first mating (62 from last).
Interestingly as soon as the waters broke, she was calm as you like all business,a dn got on with thigns delivering all five pups in an hour and 15 minutes.
Some years ago I had a friends bitch whelp her first puppy, after an uneventful first stage, and settle down to nurse. I knew half hour after the birth that she had secondary Inertia. My problem was convincing the Vet practice.
I brought her to the vet after two hours and they administered Oxytocin and Calcium, managing to bring on three more pups, but it was taking too long and was now a couple of hours after closing time.
I insisted they do a C section for the remaining pups, and she had the last three that way.
I knew that she simply had not the energy to bring them on.
She had unfortunately had Gastro enteritis at five weeks in whelp, and was off her food when she came to me to be whelped and I had to hand feed her, and she was far thinner than I was happy with, but could do no more than get as much food into her as I could.
What this long post is about is that there is that there is rarely an average whelping, what may be normal but long for one can be a danger sign for others. A bitch may quickly be in trouble or pups can come with quite long gaps perfectly healthily.
Most pups are born within an hour of their siblings but pups can come one after the other, or several hours apart. I have even had a bitch whelp each pup at 4 hourly intervals, or thought one had finished, and have another healthy pup 8 hours later.
I also know of a bitch who had a C section whelp a healthy Mum the next day!!! Sadly a poster here had missed pups on a C section born later all dead.
Nothing beats an experienced breeder of your breed and especially yoru bitches lines.
Few vets are experienced with whelping or breeding. Most only know how to perform a C section, not necessarily know exactly (to be fair no-one can as what is normal for one birth is trouble and too long for another) when one should be performed.
With primary intertia measuring falling progesterone levels is indicative but most UK vets don't have access to the equipment to test quickly. Few even keep in stock the in house premate kits that are good enough to show the levels have dropped below ovulation levels, which indicate labour should have started.