Copulatory tie

Canine reproduction is the process of sexual reproduction in domestic dogs.

In domestic dogs, sexual maturity (puberty) occurs between the ages of 6 to 12 months for both males and females, although this can be delayed until up to two years of age for some large breeds. Pregnancy is possible as soon as the first estrus cycle, but breeding is not recommended prior to the second cycle. As with other domesticated species, domestication has selectively bred for higher libido, and earlier and more frequent breeding cycles in dogs than in their ancestors.

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.

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.

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.

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 a pregnant female, and 60 to 100 days in a non-pregnant female. 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.

...
Wikipedia