Which estrogen dominates during pregnancy?

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During pregnancy the placenta becomes the main source of estrogen, and the most abundant estrogen in maternal blood is estriol. This arises because estriol is produced from fetal precursors supplied by the fetus (via the fetal adrenal and liver) and then processed by the placenta. As a result, estriol levels rise markedly as gestation progresses, especially in the third trimester, making it the dominant estrogen. Estradiol and estrone are still present but in comparatively smaller amounts. Dehydroepiandrosterone is just a precursor steroid feeding into estrogen synthesis, not the estrogen that predominates.