What does estriol mean?
Estriol is one of the three major naturally occurring estrogen hormones in humans, alongside estradiol and estrone. It is a relatively weak estrogen chemically, but its clinical importance lies overwhelmingly in pregnancy: the placenta produces it in large quantities from precursors supplied by the fetus, making it the dominant circulating estrogen of late gestation. Because fetal and placental health directly affect estriol production, clinicians measure maternal blood or urine estriol levels — historically in triple or quad screens — to help assess fetal well-being and detect certain complications. Outside pregnancy, estriol circulates at very low levels. The word derives from the Greek root underlying 'estrus' combined with the '-ol' suffix denoting an alcohol-bearing compound, reflecting its classification among steroid hormones. Though rarely heard outside medicine, estriol occupies a notable place in endocrinology and prenatal care, where its measurement has long served as a window into the health of both mother and developing child.
nounA weak estrogenic steroid hormone produced chiefly by the placenta during pregnancy, one of the three major naturally occurring estrogens in humans.
- A weak estrogenic steroid hormone (C18H24O3) produced mainly by the placenta during pregnancy and in small amounts by the liver; measured clinically as an indicator of fetal well-being.
"During pregnancy, the placenta produces large amounts of estriol, making it the dominant circulating estrogen in expectant mothers."
"Maternal serum estriol levels are routinely measured as part of prenatal screening."
"The laboratory report showed low unconjugated estriol, prompting further fetal monitoring."
Rarely pluralized; used only when referring to distinct preparations, analogues, or measured samples of the hormone.
"Several synthetic estriols have been investigated for therapeutic use."
Of the three main human estrogens, estriol is the weakest — yet during pregnancy it becomes the most abundant, surging to levels hundreds of times higher than before conception.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of estriol
The term estriol was coined in the early twentieth century following the isolation of the estrogen hormones, combining 'estra-' from Greek 'oistros' (frenzy or gadfly), the root also behind 'estrus' and 'estrogen', with the suffix '-ol' indicating an alcohol group in a chemical compound. British usage often retains the older spelling 'oestriol' with the diphthong preserved. Its coinage parallels those of its sibling hormones estrone and estradiol, all named on the same pattern.
How estriol is actually used
A technical medical and biochemical term used almost exclusively in clinical, obstetric, and scientific contexts rather than everyday conversation. In British spelling it is often written 'oestriol'. It is typically uncountable when referring to the hormone generally, though countable when referring to individual preparations or doses.
Easily confused with estriol
Estradiol (E2) is the principal and most potent estrogen of the reproductive years, whereas estriol (E3) is a weaker estrogen that predominates during pregnancy.