estriol

Estriol is the estrogen of pregnancy. Outside of pregnancy it is barely noticeable, but once a baby is on board its levels climb dramatically, making it a kind of chemical fingerprint that a placenta and fetus are actively working together.

It is the weakest of the three main human estrogens, but during pregnancy it becomes the most abundant. Its production is a true collaboration: the fetal adrenal glands and liver supply precursor steroids, which the placenta then converts into estriol. Because both partners are needed, rising estriol reflects the health of the whole fetal-placental unit, not just the placenta alone.

Estriol helps prepare the uterus and breasts and contributes to the increased blood flow of pregnancy. Historically, low maternal estriol was used as a warning sign of fetal compromise, and a low unconjugated estriol remains one component of certain prenatal screening blood tests.

A caveat: estriol measurement has largely been replaced by ultrasound and other monitoring for assessing fetal wellbeing, because it is variable and nonspecific on its own. Its main modern role is as one marker within multi-analyte screening, not as a standalone test.