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.
Small breed dogs typically start having estrus (heat) cycles earlier, at 4-6 months of age, and large breeds typically start later, at 12 to 24 months of age. The estrus cycles vary between each individual dog, average of 7 to 10 days. Some dogs as few as 2 days, some dogs cycle as long as 21 days.*.