Even a previously kind and laid-back dog may become more aggressive when she has young puppies. This is often called maternal aggression. It happens right after the puppies are born.
The reason is that the mother knows she has to provide these puppies with food, warmth, stimulation and everything else they need to live. It is her job to protect them. They are very vulnerable at this age.
The dog takes this job very seriously, and it can override her other training. If the mother believes that something is a risk for the newborn puppies, she may lash out angrily in an attempt to keep them safe.
For instance, if a young child runs over unexpectedly and tries to pick up one of the puppies, just wanting to play, the mother could then bite the child to protect the puppy. It's a natural instinct and it happens very quickly.
How can you avoid this? The best thing is just to keep your distance. Understand that the dog needs a safe space and offer her one. Keep visitors to a minimum. Do not make sudden movements that may appear threatening. After a few weeks go by, new mother dogs tend to relax a bit more. They know that the puppies have made it through this vulnerable stage in their lives. The odds of a bite go down.
Have you been bitten by an aggressive dog, or has one of your children suffered serious injuries? If so, you need to know if you have a right to financial compensation for medical bills and all of your other costs.