Objective: The purpose of this study was to evaluate the relationship between serum concentrations of relaxin and soluble CD163 with cervical length and preterm delivery in women with previous spontaneous preterm delivery. Study design: Sixty-one of 69 pregnant women with a previous spontaneous preterm had serum relaxin and soluble CD163 measured at week 16 (range, 12-25 weeks). End points were cervical length and gestational age at delivery. Results: Of the 61 women, 26% had >1 previous spontaneous preterm delivery; 84% were black; 87% were unmarried; 13% were smokers, and 39% were delivered before 37 weeks of gestation. Neither relaxin (median, 368 ng/L; range, 83-1493 ng/L) nor soluble CD163 (2.4 mg/L; range, 0.86-6.85 mg/L) correlated with cervical length or gestational age at delivery. Black women had higher relaxin levels (436 vs 205 ng/L; P = .002), but soluble CD163 levels were similar among racial groups. Conclusion: Relaxin and soluble CD163 measured at 16 weeks of gestation are not clinically useful predictors of short cervical length or preterm delivery in women with a previous spontaneous preterm delivery. © 2006 Mosby, Inc. All rights reserved.