Estriol is a type of estrogen and is a product of estradiol metabolism. The free estriol in the serum of pregnant women is synthesized by the fetus and placenta. The concentration of free estriol increases with each week of gestation and decreases rapidly after delivery of the fetus at full term. The measurement of free estriol in maternal blood can be used to determine whether the placenta is functioning normally and to predict the fetal well-being in the mother’s body, and can be used for mid-term Down’s syndrome screening in conjunction with the measurement of alpha-fetoprotein and chorionic gonadotropin levels. Increased free estriol concentrations can occur in patients with overdue pregnancies and cirrhosis of the liver. Decreased free estriol concentrations may be seen in intrauterine growth retardation, high-risk pregnancies, and placental insufficiency.