There is only certain things you can use. I have psoriasis but I have gone the oposite way and my is getting better. Stuff like exorex you can't use but best to speak to you doctor before you use anything.
My mom has psoriasis and it was exacerbated really badly with both, myself and my brother. My mom was told that she can't use most rx topicals as they cross the skin barrier. She had to cut out many citrus fruit and tomato/ tomato based products. Moisturizing with natural oils and getting sun rays worked best for her.
Stuff like epiderm ect is ok. Go see your GP if it's bad tho, they will prescribe something that is safe like a topical steriod which shouldn't hurt your baby as not much goes into your blood, but don't use anything medicated that you used before being preg without checking with a doc! Best of luck!