The male dog mounts the female from behind and inserts his penis. The penis then swells and locks into the female's vagina. The process typically lasts somewhere between five and 20 minutes.
The male's penis is not erect when it enters the female's vagina. There is a bone in the penis called the baculum that aids in insertion. When the penis has entered, it swells and the bulbus glandis on the tip of the penis enlarges and locks into the female's vagina. The male then swings his leg over so that the pair are facing away from each other. This is when ejaculation occurs. The two can part when the penis disengorges. When mating dogs, it is best that the copulation occurs in a place where the male feels more comfortable, since male dogs are more sensitive to stress during mating, according to VCA Animal Hospitals. Do not forcibly separate dogs after the locking has occurred because they could both be injured. Blood tests can indicate when a female dog is most fertile. This period is typically between the 10th and 14th day of estrus, but these times can vary, and only a blood test can tell exactly when the female dog is ready to go into heat.