If you are referring to the clear to white mucousy discharge, this starts occurring a week or so before delivery. If you see a blood show, a blackish green discharge or the water break, then a puppy should be born within an hour. If not, then you want to have your dog seen by your vet.
There are three stages to labor. During the first stage the cervix dilates and contractions begin. Your female will be uncomfortable, restless, pace and pant. This lasts 6-18 hours. During the second part of labor, your female's contractions begin in ernest. Her water breaks and puppies are delivered usually after 10-30 minutes of pushing. All puppies are born during this stage. The third stage of labor is after all pups have been delivered and is when a dog still has small contractions to expell fluids and/or placentas from the uterus.
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A dog’s gestational period (pregnancy) is approximately 63 days from breeding. One way of knowing when your dog with have her puppies is to take her temperature twice daily at the same time each day as her due date gets close. Keep a record of this.
A dog’s normal temperature is between 101-102.5F degrees. Her temperature may rise and fall during this time, but once her temperature falls below 99 degrees, she should go into labor in the next 24 hours. The temperature drop is due to a reduction in serum progesterone levels and signals that the dog will start labor within 24 hours.
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You may also notice her breasts swelling during this last week of pregnancy and some milk leakage. Dogs will usually become restless and increase their digging and nesting behavior during the last week. You will notice an increase in her panting as well as a mucous discharge. You may also notice some abdominal contractions or rippling as her time grows closer. Do not be surprised if your dog is very restless and vomits as few times during this last week.
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.
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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.
I hope you find this information helpful.