Estriol is a hydroxylated metabolite of ESTRADIOL or ESTRONE that has a hydroxyl group at C3-beta, 16-alpha, and 17-beta position. Estriol is the main estrogen in the blood and urine of pregnant females. During pregnancy, large amount of estriol is produced by the placenta. Serum estriol determinations reflect the status of the fetus and are used to evaluate fetal distress and placental function. Estriol increases until term, reaches a plateau at the 40th week of gestation and then declines. Isomers with inversion of the hydroxyl group or groups are called epiestriol. It is used in the treatment of osteoporosis in combination with Vitamin D. Its expression also seems to be one of the direct causes of preterm labour, together with cervical ultrasound mesurements and espression of Foetal Fibronectin.