Female dogs cannot get pregnant at any time. Most dogs can conceive only for several days twice a year. Much depends on the size and age of the dog. Smaller dogs might come into season three times annually, while the heat cycles of larger breeds and older canines might not occur semiannually.
Canine Heat Cycle
The canine heat cycle consists of four phases. During the initial phase, proestrus, which lasts about nine days, the female dog can attract males, but she won't permit them to breed. During proestrus, there's a bloody discharge from her vagina. During estrus, the following nine days, the female is fertile and allows males to mate. She usually ovulates within 48 hours after entering estrus. During diestrus, which lasts up to three months, her reproductive organs are concentrating on pregnancy. If she's not pregnant, she refuses to breed. Some females experience false pregnancy during this phase. The final phase, anestrus, lasts three to four months, with no sexual interest on the part of the female. If she's delivered and nursed puppies, this is the time her body repairs itself from the stress of pregnancy and lactation.
Canine Sexual Maturity
Most female dogs experience their first heat cycle at the age of 6 months. Veterinarians generally recommend spaying a dog before that date. Large breed female dogs might not have an initial heat cycle until the age of 18 to 24 months. Male dogs generally can impregnate females by the age of 6 months, although full sexual maturity occurs around a year old. Unlike females, males can breed anytime. The American Kennel Club does not recommend breeding a female dog on her first heat. The AKC won't register a litter from a female dog younger than 8 months old or a male dog younger than 7 months of age.
When your dog goes into heat, keep an extremely close eye on her to avoid an "oops" pregnancy. When she urinates during her cycle, she's sending hormonal signals to male dogs that she's available. That means you could find loose male canines attempting to get into your yard or wherever your dog happens to be. When dogs mate, the session often ends with a coital tie. After the dog ejaculates, he and the female remain stuck together, because the swollen bulbus glandis on his penis prevents them from separating. The dog might partially dismount, so the two dogs are facing opposite directions but joined at the genitals. Eventually -- perhaps in a few minutes or an hour -- the dogs naturally separate.
Increasingly, purebred dogs become pregnant via artificial insemination. The use of frozen semen allows animals separated by geography to produce offspring, and even allows male dogs to continue siring puppies long after their death. Use of fresh semen requires the presence of the male and female at the time of semen extraction and insemination, according to AKC regulations. Experienced breeders can perform the insemination themselves, while less experienced breeders should rely on a veterinarian's assistance. Artificial insemination is commonly used in breeds in which natural mating is difficult, such as the bulldog. It's also used when breeding dominant females, who might attack the male, or when the sire is an aged male.