The more babies a woman is carrying, the more likely it is that her uterus will swell, triggering contractions, says Brian Brost, M.D., associate professor of maternal/fetal medicine at the Mayo Clinic, in Rochester, Minnesota. An early due date isn't the only challenge to having multiples.
Dogs do not menstruate like humans do. Instead, they come into estrus (come into heat/season) about twice a year, once they have reached sexual maturity. This can sometimes be associated with a small amount of blood from the vulva, but it is not truly a period. Female puppies can reach sexual maturity potentially as early as six months old, although the first season (which is the closest equivalent to getting their period) is often at about nine months old.
Other End of the Spectrum. At the other end of the spectrum, puppies from young ages also require interplay with human beings, whether they've found their forever homes or not. Wee doggies who are from 8 to 16 weeks in age need to be around people -- and to be touched -- to ensure positive relationships into adulthood. If a puppy gets to 16 weeks in age and has only been around his doggie family, a strong chance exists he might take on an overly apprehensive temperament around people.