Normal range of complement components during pregnancy: A prospective study

The complement system plays a key role in normal placentation, and delicate regulation of complement system activation is critical for successful pregnancy. Therefore, establishing a normal range of complement components during pregnancy is important for clinical evaluation and research.

We performed a prospective study to investigate the normal range of complement components in circulation during different stages of pregnancy. Plasma concentrations of complement factor B (CFB), C1q, complement factor H (CFH), C3, C3c, and C4 were measured using an immunoturbidimetric assay; mannan-binding lectin (MBL), C3a, C5a, and soluble C5b-9 (sC5b-9) levels at different time points of pregnancy were determined by enzyme-linked immunosorbent assay (ELISA).

A total of 733 plasma samples were collected from 362 women with a normal pregnancy and 65 samples from non-pregnant women. In the first trimester of pregnancy, the levels of CFB, CFH, MBL, C3c, C4, and C3a were 414.5 ± 85.9 mg/L (95% CI for mean: 402.4-426.6 mg/L), 381.0 ± 89.0 mg/L (95% CI for mean: 368.5-393.6 mg/L), 4274.5 ± 2752 ng/mL (95% CI for mean: 3881.1-4656.4 ng/mL), 1346.9 ± 419.8 mg/L (95% CI for mean: 1287.7-1406.0 mg/L), 357.4 ± 101.8 mg/L (95% CI for mean: 343.0-371.7 mg/L), and 182.5 ± 150.0 ng/mL (95% CI for mean: 186.9-229.1 ng/mL), respectively. The levels of C3 and C4 increased gradually throughout pregnancy. The levels of C1q, C5a, and sC5b-9 in the first and second trimesters were nearly the same as those in non-pregnant women.

The results of this study show that pregnancy itself may influence the plasma levels of complement system components.