Sexual cycle of the bitch:
The mating period usually occurs twice a year, although in some cases the interval may be shortened or extended, having animals that cycled once every twelve months.

The sexual cycle of the bitch is divided into four stages: proestrus, estrus (these two are the heat), metestrus and anestrus each of these periods has its own characteristics.

The average proestrus lasts nine days, there are losses by bloody vulva (no analogy with menstruation in women, as human is the end of the cycle with the collapse of the lining of the uterus, while the dog is beginning cycle by increased blood flow), the labia are enlarged and hardened as we move forward in proestrus are losing turgor.

Some dogs tend to urinate more often or in places that did not usually escape or other press their regular walks.

During mating the female attracts males because in the vaginal are produced substances called pheromones, chemicals that stimulate olfactory receptors of the male, this substance may also stimulate other females with whom they live together and begin to cycle. While it attracts males, allowing the smell but not left to serve.

The onset of estrus or heat itself is marked by the acceptance of the male by the female, the duration of this stage is variable 5 -10 days. The bleeding continued, if less intense, more edema and vulva softer, if we hand over the spine at the base of the tail raises the labia, characteristic posture of acceptance, beginning the fertile period allows the service (one or miscellaneous), after this time will reject the male.

All these morphological and behavioral changes are due to the action of hormones, secreted by the ovaries, estrogen and progesterone. During the proestrus grow in the ovaries, follicles (structures that contain eggs) the more they grow, produce more estrogen (vaginal secretion, enlargement of the vulva, behavior) near the end of proestrus these hormones reach their maximum progesterone concentration begins to increase after estrogen to decrease, this decrease with the rise of progesterone allows the release of another hormone, LH (luteinizing hormone, secreted by the pituitary gland located in the cranial cavity), onset of estrus, LH will allow the release of an egg from the follicle (ovulation) about 48 hours. post-discharge. Ovulation occurs in the ovary forms the corpus luteum producing large amounts of progesterone.

The egg canine has the peculiarity that, as it is removed can not be fertilized needs to mature in a span of 48 hours.

In line with what we observe symptomatic served a bitch on day 11 after the start of the loss and get pregnant one service. However, there are jealous bitches with shorter or longer, including those that despite being at the right time do not accept rides or that acceptance begins several days before ovulation or continue to accept several days later, thereby very early or late services were infertile.

For bitches who do not accept the ride is very important to correct genital checkup and locomotion (hind legs and spine) and physical impairments that may exist for intercourse, although there were changes in behavior, females very spoiled or living with other males and become selective.

Because of these characteristics determine the fertile time the symptoms can only bring the design flaws. There are now several studies: test or dosage of LH, Pap smear test or dosage of progesterone, which allows us to locate the best time to perform the service.

The service or coupling has several stages, beginning with games and active participation of male and female, sniffing and licking of the genitals, the male mounts and embraces the female, starts the search by movements of the pelvis, then the penetration occurs, the same is achieved without an erection as the penis has a bone that confers rigidity to it.

After the penetration, copulatory movements are more intense, resulting in erection and budding.

Arriving this stage some males being dismantled and rotate both warring groups, others are mounted on the bitch like resting for a short time.

During the first minutes of this phase in which the dogs away or dismantled during the passive phase the dog mounted, ejaculation occurs the sperm-rich portion.

The rest of the time it takes the budding ejaculation occurs in the prostatic portion that gives volume to the ejaculate. The sperm come fast to the place of fertilization (fallopian tubes), it does not matter the time of docking, or the fact that the female urinate after the service.

Within 10 days after the service down to the uterus and embryos together at the same stage of development (if you received multiple services).

You're on the other stage of the cycle: metestrus, a period that begins with the end of estrus and lasts throughout the duration of the corpus luteum, regardless of whether or not pregnancy has occurred. Metestrus duration is variable, 110 to 140 days, occur in it, the implantation of embryos gestation and lactation.

In the case of the dogs that have not been served, due to high progesterone levels, may show signs of pseudopregnancy. The action that produces progesterone on the uterus to allow the puppies also host becomes susceptible to the establishment of uterine pathologies.

The last time we still have to mention is that the duration of the anestrus is variable and which will condition the dog to submit one or two cycles per year, as previous periods have a fixed average time. It is a period of sexual rest, no hormonal changes, the female has no particular symptoms nor changes in behavior.

Many times the owners wonder when it is appropriate for your pet have puppies, the right age for the first service varies by race. So, that in smaller breeds, this can be done about the year and large, it is advisable to wait up to two years. However, it must take into account only age but also the physical development and maturity appropriate for the female to maintain pregnancy, childbirth and breastfeeding puppies.

Moreover, account should be taken up when serving a bitch. While the dogs cycled all his life the past jealousy can be no external manifestations or irregular.
It would be appropriate not to extend services beyond the seven years though, this is also closely related to the state of the animal and its health and reproductive history.