Your body actually releases a lot of different hormones right now, one of them called hcg. Hcg is what pregnancy tests pick up and give you a positive. The higher your hcg levels are the further along you are in your pregnancy. Doctors draw blood and look at the amounts of the hormones to see where you are in your pregnancy among other things. Hormone levels can indicate a miscarriage if they are to low as well. Doctors also test you for all sorts of things and read your levels of hormones/chemicals/iron that's in your blood! You would honestly need to ask your doctor about it because there are so many hormones and chemicals and things they look at when they draw your blood!!
It is down from a blood test. Your body releases a chemical into your blood stream while pregnant and laboratory testing can measure the amount. Most women dont get these tests unless they are high risk for miscarriage.