Free estriol, also called unconjugated estriol, is a form of estrogen that is produced by the fetus through metabolism. It is the major estrogen in pregnancy, with relatively large amounts being produced in the placenta. Estriol levels start to rise in the eighth week of pregnancy and continue to rise until shortly before delivery. It is tested along with alpha-fetoprotein, human chorionic gonadotropin (hCG), and inhibin-A, as part of the triple test and quadruple test for antenatal screening, to assess the risk of carrying a fetus with certain abnormalities, such as Down syndrome, or Edward’s syndrome. Measurement of free estriol is a more useful indicator than total estriol, due to its short half-life in the maternal circulation. ABNORMALLY LOW estriol level is associated with Down syndrome or Edward’s syndrome.