The estriol unconjugated (E3) test is a blood test used to measure the levels of estriol in the body. Estriol is a type of estrogen hormone that is produced by the placenta during pregnancy. The E3 test is commonly used to monitor the health of the fetus during pregnancy and to detect certain medical conditions, such as Down syndrome and other chromosomal abnormalities. High levels of E3 may indicate a healthy pregnancy, while low levels may be a sign of fetal distress or other complications. However, the E3 test is not always reliable and may produce false-positive or false-negative results, so it is often used in combination with other diagnostic tests. Additionally, the E3 test may also be used to monitor estrogen levels in postmenopausal women and to help diagnose certain medical conditions, such as ovarian tumors and adrenal gland disorders.