This info is from http://www.hilltopanimalhospital.com/whelping1.htm
SIGNS OF ONCOMING LABOR
There may be a pre-labor period 8 to 24 hours in duration. The following signs may be seen during the pre-labor period, indicating the approaching whelping.
1. The bitch becomes restless, getting up, lying down, and changing her position frequently. She may vomit from nervousness.
2. She may paw and scratch at her bedding as if she were preparing a nest. She may tear newspaper up into little pieces in her attempt to make a nest.
3. Lack of interest in even the most tempting food is usually a sure sign that whelping is approaching.
4. Rectal temperature, taken with a rectal thermometer, will fall below 990 twelve hours or less prior to whelping. If’ you start taking the bitch’s temperature twice a day after the 59th or 60th day of gestation, her temperature will begin to decrease from a normal of 1010-1020 to 990-1000. When it finally goes below 990, she will start giving birth within 12 hours.
5. Milk can be expressed from the nipple near or at the time of birth in those bitches having their first litter and about 4 days prior to whelping in those bitches which have had one or more litters previously.
