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.
Most dogs are considered puppies for up to two years of age, though puppyish behavior may end sooner or last longer in some breeds. The stages listed below are essential and fairly constant. However, dogs are open to new knowledge and training well beyond the puppy years. A puppy needs occasions to meet other pets and people during this stage. 2  By three to five weeks, play becomes important as a puppy becomes aware of his or her surroundings, companions (both people and dogs), and relationships.