<chemical> (16 alpha,17 beta)-estra-1,3,5(10)-triene-3,16,17-triol. A metabolite of estradiol and usually the predominant oestrogenic metabolite in urine. During pregnancy, large amounts of estriol are produced by the placenta. It has also been obtained from plant sources. The 16 beta-isomer has also been isolated from the urine of pregnant women.