Is is possible for a female dog to get pregnant after the first intercourse?

Every dog is different and individual. The general rule is that it is during the 2nd week that a female will accept a male and allow breeding.

However, some dogs will allow it during the first week of their cycle, so yes, it is possible.

If you do not want your dog bred, she needs to be kept away from any and all males for the full 3 weeks.