hCG levels in blood and urine

The hCG (human chorionic gonadotropin or human chorionic gonadotrophin) is a hormone that is released into the woman's blood after the baby implants into the lining of her uterus. Levels of hCG are measured in the blood or urine. Most pregnancy tests are based on showing the presence of hCG in the urine (values above 20 mIU/mL).

hCG Levels During Pregnancy

The hCG level in blood is increasing about 8 to 12 days after conception, maximum values are 50-80 days after last menstruation period.

Table of hCG values (source Wikipedia):
3 weeks since LMP: 5 - 50
4 weeks since LMP: 5 - 426
5 weeks since LMP: 18 - 7,340
6 weeks since LMP: 1 080 - 56 500
7-8 weeks since LMP: 7 650 - 229 000
9-12 weeks since LMP: 25 700 - 288,000
13-16 weeks since LMP: 13 300 - 254 000
17-24 weeks since LMP: 4 060 - 165 400
25-40 weeks since LMP: 3 640 - 117 000
(twins hCG levels are higher than listed ones, see below)

Non-pregnant females: <5.0 mIU/ml 
Postmenopausal females: <9.5 mIU/ml
Tumor Marker normal values: 0-10 mIU/ml

Twin hcg levels 

The chart below shows higher hCG levels for twins. Values are based on a study (Jovanovic 1977) that studied nine multiple pregnancies (twins). Source: Book by Anne Frye - Understanding Diagnostic Tests in the Childbearing Year.