Serum relaxin in the newborn is not a marker of neonatal hip instability.

It has been suggested that intrauterine exposure to high levels of relaxin causes hip instability in newborns. The aim of this work was to evaluate whether the serum relaxin concentration in umbilical vein blood is associated with hip instability in the newborn. Blood was collected by cordocentesis from 2,185 newborns, and serum relaxin levels were obtained by using a standard sandwich enzyme-linked immunosorbent assay (ELISA). Movement of the femoral head in the hip joint was determined for all 2,185 children by the anterior-dynamic ultrasound method. Hereby their status on neonatal hip instability was determined. Fifteen cases (12 girls, three boys) were found, and 106 controls were selected. Only six newborns had unstable hips to a degree requiring treatment. Ten newborns had Ortolani-positive hips. Only three specimens of the 121 samples measured had serum relaxin concentrations above the detection limit of 10 pg/ml. None of these were cases. This study does not show an association of serum relaxin with neonatal hip instability. It is suggested that detectable serum relaxin levels are found in samples from the umbilical cord only when these are contaminated with maternal blood.