<P> The average length of the reproductive cycle for females is 2--4 weeks . Females reach sexual maturity (puberty) between 8 and 18 months of age . There is a tremendous variability in the maturation age between breeds, and even within a breed of dog . </P> <P> The first stage of the reproductive cycle is proestrus, in which eggs in the ovaries begin to mature and estrogen levels begin to rise . During this stage males are attracted to non-receptive females . Initially, the vulva lips will swell up and become pliable and there will be small amounts of bloody vaginal discharge along with signs of frequent urination and restlessness . Proestrus generally lasts 9 days . </P> <P> Estrus is the next stage, in which estrogen levels are high, mature eggs are released from ovaries, and the females mentally and physically become receptive to copulation . It is only during estrus that copulation will result in pregnancy . </P> <P> During proestrus and estrus, females may have a clear to bloody discharge . This stage is also known as "heat ." The length of these cycles varies greatly between individuals . Proestrus and estrus can last anywhere from 5 days to 21 days . </P>

When do puppies usually have their first heat