Does Locking In Always Lead To Pregnancy?

Why do dogs lock on when having sex? Does this mean the females get pregnant? Do they get hurt? (Peter Woodlock - Nyngan)

Dogs, and some other species of mammals, 'lock-on' when mating. After entering the female, the male's penis becomes further enlarged so as to prevent both the female from running away and to prevent other competing males from interrupting the process. This does not hurt the male or the female, but interruption or attempts to separate the pair may cause pain and lead to injury. Sometimes the process can take up to 30 minutes, so owners have to be patient.

As to whether the female becomes pregnant is no different to when other pairs of mammals mate. The female must be in the correct period of her oestrus cycle (which she is likely to be if she is attracting males and is receptive to them). The outcome also depends on all the variables in common with all other mammals.