Serial relaxin concentrations in human pregnancy.

Relaxin is a peptide hormone produced by the corpus luteum during pregnancy in human beings. There are no extraluteal sites of production of relaxin in women. By means of a heterologous porcine radioimmunoassay, serial serum relaxin concentrations were determined in three women throughout pregnancy and into labor. All three demonstrated a first-trimester elevation followed by a second-trimester decline; values then remained stable throughout the second and third trimesters and into labor. This further confirms the functional activity of the corpus luteum throughout pregnancy and the value of relaxin as an index of luteal function during gestation.