A molecular model of human chorionic gonadotropin, a glycoprotein hormone produced by the placenta during early pregnancy. HCG helps maintain the corpus luteum during this stage, allowing it to secrete progesterone, thickening the lining of the uterus. During a pregnancy test, HCG levels are measured in the blood or urine. These levels are also measured for fetal chromosomal abnormalities or birth defects.