Rash and testing your bile?

Im 40 weeks pregnant and have this itchy rash on my belly and thighs. It's like red spots. It's itchy most of the time but sometimes it's not, the heat makes it worse. Sometimes its more down my legs but it goes away, whereas my belly and thighs stays there even if it doesnt itch. Doctor wants me to do a blood test tomorrow just to make sure its nothing bad, and they want to test my liver and something about testing my bile. Just wondering how do they test bile? Is it just a blood test or another way?