Dogs that are neutered can still mate; however, assuming enough time has passed since the neutering, pregnancy does not occur. If the operation was recent, pregnancy could occur.
Neutering does not prevent dogs from having an erection. The operation only eliminates the formation of sperm. The testosterone level in the dog is lowered but not completely eradicated, which can cause it to have the same urge to mate as dogs that are not neutered.
This low level of testosterone leads to undesirable behaviors, such as attempting to mount, thrusting on stationary objects and excessive urination. Dogs who are mature and have only recently been neutered may still have sperm for up to a month. This increases the likelihood of impregnating a dog in heat.