I had an extremely itchy rash when I was about 18 weeks pregnant which started on my stomach and migrated down my thighs. The itching was unreal. So I became a bit of an expert on itchy rashes. Theses are a couple of rashes that can cause this in pregnancy
1. PUPPP - which basically means an itchy rash with raised bits. This is completely harmless though highly irritating. I think this is what I had and it eventually resolved itself though it took a few weeks - which is no help when you are itching like mad. This rash usually occurs in the third trimester beginning around stretch marks but my doctor rang a skin specialist and she said she had seen it in earlier in pregnancy also. I used calamine, pinetarsol and eventually got steroid cream prescribed. Honestly calamine was as good as anything. Antihistamines did nothing and I didn't really like taking them anyway. Occasionally in really bad cases they prescribe steroid tablets which are very effective and relatively safe. But its a case of the benefits outweighing the risk.
2. There is another rash called intrahepatic cholestasis which is more serious, though rare. It usually shows up in more strange places like the soles of the feet. It has something to do with the liver and can be easily ruled out by a liver function test -which I had just to be sure. It can be harmful to the baby and if this is the rash then the baby will need to be monitered more closely, so it is worth getting this simple test.
Both of these have loads of info on the internet
I am pleased to say that once it went away it never came back - which was my biggest fear.
Good Luck - hope this helps
Leanne, NZ, Ella age 2 & Sam age 6months