Just make sure the dandruff shampoo is okay'd by your dr. I know, crazy, it's just shampoo right? But personal care products can have things in them you shouldn't use, esp during first trimester. Motherrisk at Sick Kids can also help.
Have you tried dandruff shampoos? Maybe washing your hair less often would help. Pregnancy can cause some crazy changes during and after, my hair for instance I can wash it and pull out a hanful of hair. I shed worse than an animal! Lol.