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.
This is the average age of puberty and may vary according to the breed of the dog. For small breeds, this process starts at around 5-6 months of age. While for the large breeds, it can start when the dog is about 2 years old. Female dogs normally have only 2 reproductive cycles in a year. During the cycle, they will experience bloody vaginal discharges that last for 4-9 days. After this duration, the female dog will begin to stand for breeding and accept the male.