Frankly all the Dandruff shampoos have Ketoconizole which is contraindicated in the first trimester.
Since the shampoo is for external application it can't get into the circulation . So no harm can come to pregnancy when you are in the second trimester.
But , why are you so keen on shampooing when you are
pregnant?
You already have apprehensions.
It is better to wait for another four months. After all pregnancy is not a "life long disease" . It will be over in another four months.