The average length of the reproductive cycle for females is 6 months. Bitches reach sexual maturity (puberty) between 4 and 18 months of age. There is a tremendous variability in the maturation age between breeds, and even within a breed of dog. 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 vulvar lips will swell up and become pliable, there are small amounts of bloody vaginal discharge, and signs of frequent urination and restlessness. Proestrus generally lasts 9 days. Estrus is the next stage, in which estrogen levels are high, mature eggs are released from the ovaries, and the females mentally and physically become receptive to copulation. It is only during estrus that copulation will result in pregnancy. 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. Diestrus is the period following mating. Diestrus lasts approximately 56 to 58 days in the pregnant females, and 60 to 100 days in the non-pregnant females. During both of these periods, progesterone levels are high. Because the hormonal profile of a pregnant female and a female in diestrus are the same, sometimes a non-pregnant female will go through a period of pseudo-pregnancy. At that time she may gain weight, have mammary gland development, produce milk, and exhibit nesting behaviours. Anestrus is the period of reproductive quiescence. The female has no attraction to or from the male. Anestrus generally lasts four to five months.