Complement activation in fetuses: assessment by the levels of complement components and split products in cord blood.

The whole complement activity (CH50) and concentrations of components (C1q, C3, and C4) and C3 split product (C3d) were determined in the cord blood of 172 newborns between 16 and 42 weeks of gestation. Histological examination of their placentas revealed that the extent of complement activation was significantly higher in those with some degree of inflammation in the amnion than in those with normal amnion. The correlation between the complement levels and gestational age was higher in those with no amnionitis. In some cases the level of C3d at an early gestational age was as high as that in adults. The C3 activation system seems to be well developed even before week 25 of gestation. Among the complement components, C3d was the most sensitive indicator of placenta inflammation. As placenta inflammation may reflect intrauterine infection, the measurement of C3d levels may be of diagnostic value.