The external genitalia of a male dog consists of two testes contained in an external scrotal sac. The external orifice of the penile sheath is located just caudal to the umbilicus. The female's external genitalia consists of a vulva. Estrus lasts for two or three weeks and occurs at approximately six-month intervals. The start of estrus is signaled by vulvar swelling and a blood-twinged vaginal discharge.
Ovulation is spontaneous and takes place during early to mid-estrus.
The female is attractive to males during the entire estrus two-three week period, but usually permits mating only during a four to ten day window, beginning eleven to seventeen days after the onset of full estrus. After some exploratory sniffing and licking by the dog, the bitch signals her readiness to mate by standing with her tail held to one side. The dog mounts the bitch, clasping her around the loins with his forelegs. He makes thrusting motions and releases his semen within a minute. Often the male will turn so that the dogs are partially back to back.