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.
While most pups are considered adults when they reach 1 year old, the fact is, dog breeds mature at slightly different rates, and not all dogs are full-grown on their first birthday. Small breeds reach adult size more quickly; some larger breeds may not be fully grown until around 2 years of age. Until they are fully mature, pups need special nutrition to encourage healthy growth at an appropriate rate.