Estimation of oestrone sulphate in the serum of pregnant sows

Oestrone sulphate was extracted from pig serum with tetrahydrofuran and then hydrolysed with sulphatase. The radioimmunoassay of liberated oestrone was carried out directly on the reaction mixture. Oestrone sulphate concentrations were less than 60 pg/ml in non-pregnant sows. Detectable concentrations of oestrone sulphate were found in serum as early as the 18th day of gestation, and these rose to maximum levels of 2.1-3.5 ng/ml by Days 26-29 then fell to about 0.1 ng/ml by Day 37. Serum oestrone sulphate concentrations were not related to litter size, but were good indicators of early pregnancy.