Radioimmunoassay of plasma relaxin levels throughout pregnancy and at parturition in the pig.

Relaxin concentrations in peripheral plasma have been measured throughout pregnancy and immediately following parturition in the pig by radioimmunoassay. Gestation ranged from 114-117 days. Relaxin concentrations remained below 2 ng/ml during the first 100 days of pregnancy and then rose gradually to a mean of 11.9 ng/ml by 3 days before parturition. At this time, relaxin levels began to rise more rapidly and over the next 2 days increased to a mean concentration of 44.0 ng/ml by 30 h before parturition. Relaxin concentrations then increased sharply to a mean of 145.6 ng/ml by 14 h before parturition. This maximum was followed be a rapid decrease to a mean of 42.4 ng/ml by 2 h before parturition. At one day following parturition, relaxin levels were less than 1 ng/ml.