They just want to see if it's doubling in numbers, very normal 

This is my third pregnancy. I went last Tuesday, Friday, and today. My levels have all been fine. Tomorrow I will be 5 weeks. My doctor just like to make sure they are rising appropriately. 

I think it depends on the doctor and your situation.Since this was my first time being pregnant, my doctor did no blood work for HCG.  I asked about it and he said they would if I had a previous miscarriages or was considered high risk.He said some other doctors might do it for everyone but since my insurance doesn't cover blood work, he wasn't going to suggest doing it if it wasn't necessary.