Once your dog's temperature has dropped, please do not leave her alone in case of complications. As she enters actual labor you should be able to see her contractions and most dogs will begin looking at their hindquarters. An increase of licking of the genitals is normal before and during labor. Be sure and have an emergency Vet’s phone number readily available just in case of an emergency during the birthing of the puppies.
Here is an excellent site about whelping.
Breeding, Whelping, and Rearing Puppies
Some other signs that my dogs always seem to exhibit are lack of appetite, frequent urination and frequent small, runny bowel movements in the day before labor starts. Frequently my dogs will start looking at their rears when the first real contractions start.
Once you have blood show, or a black or reddish discharge, then you should expect to see a pup within 20 minutes. Most real contractions have a dog looking at their rear in a puzzled manner the first litter. In some cases, you will see the abdomen tighten as well. Most dogs will bear down as well with conractions.
Now the problem you might have is that chihuahuas frequently are unable to deliver their pups naturually due to t he narrow hips of the mom and the large heads of the pups. So many times a pup is stuck and the mother is having contractions that eventually become weak and the mom and pups are then put in danger. For this reason we do recommend that chihuahuas be x-rayed the last week of pregnancy so the vet can determine if a natural delivery is possible and count the pups so you know when labor is done.
I wished I could be more specific as to how to tell a contraction, but each dog is a bit different and a lot of interpretation on your part is necessary
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I own a chihuahua. She is 67 days pregnant. I th... - JustAnswer