Oestrone sulfate commences an increase around 50 days of gestation in bovine peripheral blood.

The objective of this study was to determine the peripheral plasma concentration of oestrone sulfate during early gestation in the cow with a sensitive assay system. Five Holstein heifers were inseminated on oestrus and bled from the jugular vein at regular intervals until 100 days of gestation. Oestrone sulfate, oestrone and oestradiol-17beta in blood plasma were extracted with a reverse-phase cartridge and each measured by specific radioimmunoassay. Oestrone sulfate in bovine circulation started to increase around 50 days of gestation, whereas oestrone and oestradiol-17beta remained at basal concentrations for 80 days. The plasma concentration of oestrone sulfate increased gradually and linearly from 50 days of gestation, and was drastically elevated after 80 days of gestation before the increase in unconjugated oestrogens. The rate of increase from 80 to 100 days was the greatest for oestrone sulfate concentration among all oestrogens. These results suggest that plasma oestrone sulfate concentration could be a useful indicator of pregnancy within the first trimester of gestation in the cow.