Distribution of relaxin between human maternal and fetal circulations and amniotic fluid.

Relaxin was measured in maternal blood and amniotic fluid samples at 9-40 weeks and in fetal blood samples at 19-41 weeks of pregnancy. In amniotic fluid, concentrations of relaxin rose from 58 ng/l (geometric mean) at 10 weeks to 142 ng/l at 14 weeks and declined subsequently to 55 ng/l at 22 weeks. In maternal blood, mean relaxin concentrations were ten times greater than in amniotic fluid, and concentrations decreased with gestation. Since there was no significant association between the relaxin concentrations in the two compartments, relaxin in the amniotic fluid may be derived from the decidualized endometrium rather than the maternal circulation, alternatively its metabolism may be different in the two compartments. The absence of detectable concentrations of relaxin in any of the fetal blood samples demonstrates that there is no significant placental transfer or fetal synthesis of this peptide.