Estrogens induce myometrial contractility. Estriol is the most abundant estrogen during pregnancy and is produced by the placenta. It first appears at approximately the ninth week of gestation from the last menstrual period (3). Plasma concentrations of this estrogen increase throughout gestation along with those of estrone and estradiol (3,4,5). In the weeks preceding parturition, concentrations of estriol dramatically increase and continue to rise until birth (6). Estriol's roles include preparing uterine tissues for contraction and possibly signaling parturition in higher primates and humans (3,4,7,8,9,10). A rise in estriol, which can be monitored in saliva, has been shown to precede parturition, regardless of whether labor begins preterm, at term, or postterm (11,12).