Unconjugated Estriol (E3) is a form of estrogen produced primarily during pregnancy by the placenta and the fetus. The Unconjugated Estriol (E3) test measures the level of this hormone in the blood, often used as part of prenatal screening during the second trimester. It helps assess the health and development of the fetus and screen for potential chromosomal abnormalities, such as Down syndrome (trisomy 21) or trisomy 18. Unconjugated Estriol levels, when combined with other tests like alpha-fetoprotein (AFP) and human chorionic gonadotropin (hCG), provide a comprehensive risk assessment for fetal conditions.