Estriol (E3)

What it isEstriol is the weakest of the three main estrogens and dominates during pregnancy, when the placenta produces it in large amounts.

DoesSupports the pregnant uterus and maternal physiology; used in some HRT formulations for vaginal symptoms.

GoodPregnancy-appropriate levels signal placental health.

BadOutside pregnancy, levels are very low and not typically clinically meaningful.