Estriol is a female hormone that exists in blood in the free and conjugated forms (primarily estriol-3-sulfate-16-glucuronide, estriol16-glucuronide and estriol-3-sulfate). Measurement of total estriol has been shown to be useful in monitoring fetal function. During pregnancy, virtually all of the estriol in the maternal circulation is synthesized in the feto-placental unit. Estriol begins to appear in the maternal circulatory system in the ninth week of gestation. During the third trimester, estriol levels in serum and urine normally increase rapidly. Therefore, a low or declining estriol level may be indicative of fetal distress. Although a single estriol measurement may have some value as a serum test, serial determinations to monitor estriol are considered more useful.