How Can U Tell When A Pit-bull Is Ready To Give Birth?

One of the ways you can tell a pit bull is about to give birth is by taking her temperature rectally. A normal dog temperature is between 100.3F to 101.3F. But eight to twenty four hours prior to giving birth this temperature will drop to 98 F. The pit bull will also decline to eat anything just when she is about to give birth.

After you had done your research on how to be a good breeder and breed responsibly (and what to expect, problems, ect.) and chosen your breeding candidates very carefully, when they were mature enough to breed (age), you carefully selected a stud according to appropriate criteria, you did your prenatal care and waited the appropriated gestation period and their temperature drops and they display some first stage behaviors and then their water breaks and pups arrive soon after. There is more detailed breeding and whelping basics information and links below for you.
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http://www.wildheirlabradorrescue.org/readingroom/what-is-a-back-yard-breeder.php  
http://members.tripod.com/antique_fcr/goodbreeder.html

Her temperature will drop below 100*... Do a search of the site, several of us have answered this in detail under similar questions recently.
If you have any questions after reading those... leave it in my "shout out" box and I'll do my best to help.