Estriol Unit Converter

Estriol (E3) is one of three major estrogen hormones in the body, alongside estradiol (E2) and estrone (E1). It is the weakest of the three estrogens, with a binding affinity for estrogen receptors roughly 11–14% that of estradiol.

In non-pregnant individuals, estriol circulates at nearly undetectable levels. During pregnancy, however, levels increase roughly 1,000-fold as the placenta produces large quantities through a pathway that depends on the fetal adrenal glands and liver.

At term, estriol accounts for about 90% of the estrogens in the urine of pregnant women. Unconjugated estriol (uE3) is commonly measured as part of prenatal screening tests to assess fetal and placental health.

Outside of pregnancy, estriol is produced in small amounts by the liver through conversion of estradiol and estrone via the enzyme CYP3A4. During pregnancy, the placenta produces estriol in large quantities using precursors (DHEA-S and 16α-OH-DHEA-S) supplied by the fetal adrenal glands and liver, with approximately 90% of precursors originating from the fetus.