Serum oestrone sulphate concentration was measured in samples taken from 1275 sows of four breeds or crossbreeds (large white, landrace, large white cross landrace and monarch hybrid) during the period 25 to 30 days after mating. A simple, direct radioimmunoassay using 20 microliter serum was employed for the estimation of oestrone sulphate and pregnancy diagnoses were made on the basis that more than 0.5 ng oestrone sulphate/ml serum indicated a pregnant sow and less than 0.5 ng/ml indicated non-pregnancy. Overall accuracy of pregnancy diagnosis based on oestrone sulphate was 98 per cent; this was not influenced by breed or day sampled (within the range tested). For pregnant sows, there was a positive correlation between serum oestrone sulphate level and litter size, although individual values could not be used to predict litter size for particular sows. Oestrone sulphate concentration was also measured in samples taken from pregnant monarch sows during the last third of the gestation period and the level was more than 0.5 ng/ml in 99 per cent of samples taken on day 77 or later. Thus, measurement of serum oestrone sulphate level in samples taken more than 76 days after mating could be used as a confirmatory test of pregnancy.
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