he says that in such circumstance her fright may turn into agression when she has pups and she might become calmer after,
Agree with others...but just reason this out!
Breed your girl and she becomes aggressive...?? How's that preferable to being afraid??
Get a new vet. Find out about anti-anxiety medication and a Thunder shirt.
Also consult a trainer. You could be making her fear worse by your actions but we can't say if that's the case without being there.