A dog carries puppies for about two months or 60 to 64 days. This is true for all breeds, though small-breed dogs typically begin going into heat earlier than large breed dogs.
Small-breed dogs typically begin estrus, or heat, cycles at around 4 to 6 months of age. Large dog breeds typically start at nine months, although some may not begin their heat cycles until 24 months of age. Cycles vary between individual dogs. Some dogs have a heat cycle that lasts as little as two days, while others may last for 21 days. The average number of days in a heat cycle is six to 12.