Dogs cannot become pregnant at any time. Female dogs come into season, or into their heat cycle, anywhere between two and three times a year, depending on the breed. Larger dogs might go into season more often, but for the most part, female dogs come into season twice a year. Male dogs are always ready to reproduce, but a female dog will not let a male dog mount her until she is ready to conceive.
A female's heat cycle is anywhere from seven to fourteen days. The first part of the cycle consists of a reddish discharge from the female and of increased attention paid to her reproductive area. During the first seven or eight days, male dogs will show an increase in the attention that they pay to her. As she approaches the time in which she will be able to conceive, she will begin to reciprocate the attention. When the female is ready, she will let the male know.
As the female begins to be ready to conceive, she will start the mounting process with the male dog. It begins with her mounting him to show that she is ready to become pregnant. After she has done this, there will be a dance, or a play-type time, in which they mount each other and in which the female presents herself to the male to be mounted. The male eventually wins this dance, and he mounts her. At this time, the male enters the female, and a "tie" occurs. The male and female dog stay tied together during intercourse for up to 20 minutes at a time. During this time, the male's semen seeps into the female's vulva. When the process is finished, the dogs disconnect.
If the female has allowed the male to mount her and a tie has taken place, unless there is something wrong with the female or unless the male has been neutered, the female will become pregnant. Many breeders allow male and female pairs to tie several times, just to be sure, but most of the time, one tie is sufficient. The gestation period in dogs is about 60 or 70 days