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 breeds may be in puberty at 6 months of age, while larger breeds may be 2 years old before entering puberty. The smaller dogs may cycle twice a year, or three times. Larger dogs may cycle every 8 to 9 months; many cycle annually. The largest dogs may go 12 to 18 months, or longer.