ABSTRACT Estrone sulfate, estrone, estradiol and estriol were measured through the course of pregnancy in 85 women. Of the unconjugated estrogens, estradiol had the highest plasma concentration, being approximately twice that of estrone and four times that of estriol. Estrone sulfate level was twice that of estradiol. All estrogens measured showed the same pattern of increase with time. When plotted against fetal and placental weights, the increase in plasma concentration of all estrogens correlated better with fetal weight than with placental weight.