Estriol is a naturally occurring estrogen hormone and is the weakest of the three main estrogens found in the human body, the others being estradiol and estrone. It is present in very small amounts in non-pregnant individuals but is produced in large quantities during pregnancy by the placenta, with contributions from both the mother and the fetus, where it helps support uterine growth, improve placental blood flow, and prepare the body for childbirth. Estriol was first identified in the 1930s during studies on female sex hormones, and researchers soon recognized its milder biological activity compared to other estrogens. By the mid-20th century, estriol measurement became important in prenatal care, as its levels were used to assess fetal and placental health, and later studies explored its potential therapeutic use due to its relatively lower risk of side effects.
