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. Estriol increases until term, reaches a plateau at the40th week of gestation and then declines. somers with inversion of thehydroxyl group or groups are called epiestriol. It is used in the treatmentof osteoporosis in combination with Vitamin D. Its expression also seems tobe one of the direct causes of preterm labour, together with cervicalultrasound mesurements and espression of Foetal Fibronectin.