The human chorionic gonadotropin (hCG) blood test measures the level of hCG hormone present in a sample of your blood. HCG is produced during pregnancy.

Human Chorionic Gonadotropin (hCG) hormone: 
The Pregnancy Hormone. The hormone human chorionic gonadotropin (better known as hCG) is produced during pregnancy.
It is made by cells formed in the placenta, which nourishes the egg after it has been fertilized and becomes attached to the uterine wall.

What is an hCG blood test for?
A qualitative HCG blood test checks if there is a hormone called human chorionic gonadotropin in your blood. HCG is a hormone produced in the body during pregnancy.
Other HCG tests include: HCG urine test.

hCG level which indicate pregnancy
An hCG level of less than 5 mIU/mL is considered negative for pregnancy, and anything above 25 mIU/mL is considered positive for pregnancy.
The hCG hormone is measured in milli-international units per milliliter (mIU/mL).

See> hCG levels chart for twins