Serum relaxin and pelvic pain of pregnancy.

Serum relaxin immunoreactivity was measured by means of a porcine relaxin radioimmunoassay in 35 patients with severe pelvic pain and pelvic joint instability during late pregnancy. Results were compared with a control group of 368 samples obtained throughout pregnancy from normal singleton pregnancies. Most of the relaxin concentrations in the study group were above the 95% confidence limits of the median for the corresponding gestational age in the control group. The difference in relaxin levels between the study and control groups in the third trimester was highly significant. Relaxin levels in patients with pelvic pain were close to normal non-pregnant levels by the third postnatal day. The highest relaxin levels during pregnancy were found in the patients who were the most incapacitated clinically. The results suggest that there may be an association between high serum relaxin levels and pelvic pain and joint laxity during late pregnancy.