Estriol, a metabolite of 17b-estradiol and a major estrogen produced during pregnancy. In the final weeks before parturition, estriol levels increase significantly (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.