I have bred many litters of spaniels and cockers over 30 years and all my bitches have been kept in peak fitness throughout pregnancy with no ill-effects to the unborn puppies.
As regards running in trials, there is only a short time when you would be able to do that. The bitch must have fully finished her season and should not be showing signs of pregnancy externally.
There is, therefore, only a three-week period of opportunity, but I would see no reason why you should not still compete with her.
I have always continued with working my bitches on shoots, but as the pregnancy progresses, the length of time and type of work they would be subjected to would be carefully monitored and reduced accordingly.
In a trial you do not have that same control and the dog would be expected to undertake all the tasks required. During oestrus and pregnancy, the bitch’s hormonal changes may also adversely affect her attitude to you and to work.
If she has an off day because of her condition and does not perform at her best in public, that would certainly not benefit her future trialling career or the marketing of the puppies.