The test is most commonly used to detect pregnancy.
It can be detected as early as 10 days after a missed menstrual period.
During the early weeks of pregnancy, hCG is important in maintaining function of the corpus luteum (the mass of cells that forms from a mature egg). Production of hCG increases steadily during the first trimester (8 - 10 weeks), peaking around the 10th week after the last menstrual cycle. This is the reason it is sometimes used to monitor pregnancy during that period.
hCG is also produced by some germ cell tumours and increased levels are seen in trophoblastic disease.