The Free Estriol ELISA kit is used for the quantitative measurement of free Estriol in human serum or plasma. Estriol (1,3,5(10)-estratriene-3,16,17-triol; E3) is one of the three major naturally-occurring estrogens produced almost exclusively during pregnancy. Maternal Estriol levels, alone and in combination with hCG and AFP, have been recommended to monitor fetal status. During pregnancy, the production of Estriol depends on an intact maternal-placental-fetal unit. Fetal-placental production of Estriol leads to a progressive rise in maternal circulating Estriol levels, reaching a late-gestational peak which is ~2-3 orders of magnitude greater than nonpregnant levels. In the maternal circulation, Estriol undergoes rapid conjugation in the liver followed by urinary excretion with a half-life of ~20 minutes. Therefore, maternal Estriol levels can provide a dynamic estimate of fetal production rates. In terms of estrogenic activity, Estriol is much less potent than Estradiol Because Estriol concentrations are subject to diurnal and episodic variation, it is common practice to refer serum measurements to a baseline, defined for the patient as either the average or the highest of her three most recent Estriol results.