Estriolis a hydroxylated metabolite of ESTRADIOL or ESTRONE that has a hydroxylgroup at C3-beta, 16-alpha, and 17-beta position. Estriol is the mainestrogen in the blood and urine of pregnant females. During pregnancy, largeamount of estriol is produced by the placenta. Serum estriol determinationsreflect the status of the fetus and are used to evaluate fetal distress andplacental function.