Estriol is a metabolite of estradiol and a major estrogen produced in the later stages of pregnancy. In a longitudinal study in healthy pregnant women, total plasma estriol levels increased from 10 weeks gestation to approximately 150 ng/ml at week 38. The majority of the estriol synthesized in the later stages of pregnancy originates from fetal dehydroepiandrosterone sulfate (DHEAS) and serves as a direct marker of fetal adrenal gland activity. Saliva contains primarily unbound and unconjugated estriol and is commonly used for monitoring estriol levels. Plasma levels of estriol in males and non-pregnant females is less than 2 ng/ml.