- Plasma concentrations of fragment Bb (FBb) are a marker for activation of the alternative pathway of the complement system. High concentrations of FBb in maternal blood, as early as the first trimester, are associated with subsequent spontaneous pre-term delivery <34 weeks of gestation. The aim of this study was to determine whether spontaneous pre-term labor (PTL) with intact membranes, intra-amniotic infection/inflammation (IAI) or labor at term are associated with alterations in circulating maternal FBb concentrations.This cross-sectional study included women in the following groups: (i) non-pregnant (n = 40); (ii) normal pregnancy (gestational age range 20-36, 6/7 weeks, n = 63); (iii) women at term not in labor (n = 70); (iv) women at term in spontaneous labor (n = 59); (v) patients with an episode of PTL who delivered at term (n = 62); (vi) PTL without IAI who delivered pre-term (n = 30); and (vii) PTL with IAI who delivered pre-term (n = 67). Maternal plasma FBb concentrations were determined by ELISA.(i) Among patients with PTL, those who had a pre-term delivery either with IAI (1.21 microg/mL, IQR 0.77-2.16) or without IAI (1.13 microg/mL, IQR 0.92-2.08) had a higher median maternal plasma FBb concentration than those who delivered at term (0.86 microg/mL, IQR 0.64-1.57; P = 0.007 and P = 0.026, respectively); (ii) there was no difference in the median plasma FBb concentration between patients with and without IAI who delivered pre-term (P = 0.9); (iii) in contrast, spontaneous labor at term was not associated with a significant change in the maternal plasma FBb concentration (P = 0.8); (iv) maternal plasma concentration of FBb did not differ significantly between normal pregnant women and the non-pregnant controls (P = 0.8) and were not correlated with advancing gestational age (r = -0.28, P = 0.8).(i) Pre-term parturition is associated with activation of the alternative complement pathway in maternal circulation; (ii) such activation is not detectable in spontaneous labor at term; (iii) IAI does not explain the activation of the alternative pathway of complement in PTL. Collectively, these observations suggest that pre-term and term labors have fundamental differences in the regulation of innate immunity.