In contrast, the canine reproductive life varies greatly from breed to breed with regard to onset of puberty. Small breed dogs typically start having estrus (heat) cycles earlier, at 4-6 months of age, and large breeds typically start later, at 9 to sometimes as late as 24 months of age.
Estrus cycles in the dog vary between each individual dog, average of 6 to 12 days. Some dogs as few as 2 days, some dogs cycle as long as 21 days.1
Female dogs cycle throughout their life, they do not experience menopause (reproductive cycles ceasing with age) as human females do.
Learn more about: the canine estrus cycle
Spay surgery Female dogs that are spayed eliminate the chance of uterine infection (a condition called pyometra), uterine cancer, and ovarian cancer, since the uterus and ovaries are removed during the spay surgery.
Additionally, the chance of breast cancer is greatly reduced, especially if the dog is spayed before the first heat cycle. The earlier that the dog is spayed, the better the chance for reducing or eliminating all of these reproductive diseases mentioned above.
1Canine Reproductive Cycle: From the Merck Veterinary Manual, 8th ed. page 980