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 wh ether 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 stan dard sandwich enzyme-linked immunosorbent assay (ELISA). Movement of the fe moral head in the hip joint was determined for all 2,185 children by the an terior-dynamic ultrasound method. Hereby their status on neonatal hip insta bility was determined. Fifteen cases (12 girls, three buys) were found, and 106 controls were selected. Only six newborns had unstable hips to a degre e requiring treatment. Ten newborns had Ortolani-positive hips. Only three specimens of the 121 samples measured had serum relaxin concentrations abov e the detection limit of 10 pg/ml. None of these were cases. This study doe s not show an association of serum relaxin with neonatal hip instability. I t is suggested that detectable serum relaxin levels are found in samples fr om the umbilical cord only when these are contaminated with maternal blood.