There is some variation in the age at which dogs reach sexual maturity. Small breeds appear to mature faster than large ones, which usually cycle later. It is not uncommon for large-breed females to come into heat for the first time at more than 1 year of age, although 8 to 9 months is the norm. Dogs are sexually mature between 6 months and 1 year but are not socially mature until they are about 2 years of age. Females first cycle anywhere from 6 to 18 months of age and approximately twice a year thereafter. The only exception is the African basenji, which cycles annually, bearing one litter a year.
The heat cycle of the female lasts from 18 to 21 days. The first stage is called proestrus. It begins with mild swelling of the vulva and a bloody discharge. This lasts for about 9 days, although it may vary by 2 or 3 days. During this phase the bitch may attract males, but she is not ready to be bred and will reject all advances. The next phase is the estrus. Usually the discharge decreases and becomes lighter, almost pink, in colour. The vulva becomes very enlarged and soft, and the bitch will be receptive to the male. This stage may last 3 or 4 days or as long as 7 to 11 days. The female may be receptive a day or two past the time when she would still be fertile. In order to be sure that the breeding is taking place at the optimum time, vaginal smears and blood tests can be done by a veterinarian beginning before estrus and through the estral phase.
At about the 14th day, or whenever estrus ends, the final, or luteal, stage of the cycle begins; this stage is called diestrus. The discharge becomes redder, the vulva returns to its normal size, and the bitch will no longer accept the male for mating. When all signs of discharge and swelling are absent, the heat is complete. The diestrus stage lasts 60 to 90 days (if no pregnancy has occurred) or until the bitch gives birth. She then enters anestrus, which is the time frame between the end of the last cycle and the beginning of the next proestrus.
Canine males are always fertile from the onset of their sexual adolescence, usually after six months of age. Larger-breed males may take a few months longer to become sexually mature. Males are usually promiscuous and are willing to mate with any available female.
Males produce far more sperm than is needed to impregnate the ova that are released during estrus. Small-breed bitches usually produce small litters. Two or 3 puppies in a breed such as a Yorkshire terrier is considered the norm. Large-breed litters can have as many as 10 or 12 puppies, although the normal bitch can suckle up to 8 at a time.
Blind and unable to stand, newborn puppies are helpless and spend 90 percent of their time sleeping and 10 percent nursing. Becoming chilled is the greatest danger facing a healthy newborn puppy because its immature circulatory system cannot sustain an adequate body temperature. For this reason, newborn puppies tend to stay close to their mother or cuddle together for warmth. Mothers clean, nurse, and defend their pups until they can live on their own, but fathers do not involve themselves in the care of the young.
Gestation and whelping
The normal gestation period is 63 days from the time of conception. This may vary if the bitch has been bred two or three times or if the eggs are fertilized a day or two after the mating has taken place. Eggs remain fertile for about 48 hours. Sperm can live in the vaginal tract for several days. In order to determine if a bitch is pregnant, a veterinarian can manually palpate her abdomen at about 25 days after breeding. Ultrasound also can be done at that time. At about 40 days X rays will confirm pregnancy.
Most bitches whelp normally. However, the large-headed, short-bodied breeds and the toy breeds often must undergo cesarean sections in order to deliver live puppies.
Both males and females are fertile well into their advanced age. It is generally considered best for the bitch to be bred for the first time upon maturity but not before her second or third heat cycle, depending on her age at the first. Because small breeds mature more quickly, they can be bred at an earlier age than large breeds. A bitch will have less difficulty in conceiving and carrying a litter if she is bred before the age of five. As she becomes older, litter size generally decreases. After the age of seven, bitches are likely to have small litters and experience problems in delivering the puppies. Veterinarians feel that bitches generally should not be bred after that age.
Males can be bred as long as they are fertile, although with age the motility and quantity of sperm decrease.
Source: "dog." Encyclopędia Britannica from Encyclopędia Britannica 2007 Ultimate Reference Suite.(2007).