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 reproductive phase in dogs and if the female dog undergoes intercourse at this time, she could get pregnant. The female dog menstruation period usually starts when she is 5 to 10 months old; however, in some small breeds, it may start at six months. The female dog menstrual cycle lasts for 21 days or three weeks on an average, but it may vary from 8 to 30 days. Most female dog heat cycle is experienced at least twice a year or once every seven months. The heat cycle or the canine estrous cycle is divided into the four stages mentioned below: