Estriol (E3), also spelled oestriol, is a steroid, a weak estrogen, and a minor female sex hormone.

[13][8][6][14] Estradiol increases breast cancer cell growth via activation of the GPER (in addition to the ER), and estriol has been found to inhibit estradiol-induced proliferation of triple-negative breast cancer cells through blockade of the GPER.

[14] In women who are not pregnant estriol is produced in only very small quantities, and circulating levels are barely detectable.

[5] As such, target tissues are likely to be exposed to similar amounts of free estriol, estradiol, and estrone during pregnancy.

[3] Then, placental 17β-hydroxysteroid dehydrogenase interconverts estrone and estradiol and the two hormones are secreted into the maternal circulation.

[3] Estriol is poorly bound to sex hormone-binding globulin (SHBG),[21] with much lower binding affinity for this protein, relative to estradiol, and hence a greater fraction available for biological activity.

In comparison, in near-term pregnant women, estriol urinary excretion ranges from 50–150 mg every 24 hours.

[26][27] The name estriol and the abbreviation E3 were derived from the chemical terms estrin (estra-1,3,5(10)-triene) and triol (three hydroxyl groups).

[28][29] Estriol can be measured in maternal blood or urine and can be used as a marker of fetal health and well-being.

Because many pathological conditions in a pregnant woman can cause deviations in estriol levels, these screenings are often seen as less definitive of fetal-placental health than a nonstress test.