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.
The gestation or pregnancy period in dogs begins from the first breeding session or fertilization until the birth of the puppies. Though the period may vary as per the size or breed of the dog, usually, the gestation for small dogs as well as big dogs lasts for about 60 to 63 days, or 9 weeks. Female dogs have menstrual cycles throughout their lives and are capable of reproducing for as long as they live.