HCG is an hormone that is secreted by the embryo. Presence of certain levels of this hormone in blood and urine indicate pregnancy. Its levels 0-5 means no pregnancy. During pregnancy hCG levels continue to double after 48-72 hours. Up to 16 week of pregnancy, hCG levels are at peak and their quantity is 13,300 - 254,000. After this hCG levels start to drop and during 25-40 weeks of pregnancy, hCG levels are 3,640-117,000.