Complement C3 (serum)
Complement C3 normally increases during pregnancy because of enhanced hepatic synthesis and physiologic activation of the innate immune system.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| mg/dL | 83 – 177 | 44 – 116 | 58 – 118 | 60 – 126 |
| g/L | 0.83 – 1.77 | 0.44 – 1.16 | 0.58 – 1.18 | 0.60 – 1.26 |
Pregnancy physiology
- Hepatic C3 synthesis increases under estrogen stimulation.
- Innate immunity is upregulated to protect against infection.
- Complement contributes to trophoblast invasion and placental development.
- Levels rise progressively across pregnancy.
Causes of decreased C3
- Systemic lupus erythematosus (SLE)
- Lupus nephritis (especially proliferative classes)
- Acute post-streptococcal glomerulonephritis
- Hereditary complement deficiency
- Sepsis or massive complement consumption
- Chronic liver disease (reduced synthesis)
- Viral infections (hepatitis, EBV, parvovirus)
Falling C3 in pregnancy is particularly concerning in SLE, where it may signal a flare or renal involvement.
Causes of increased C3
- Normal physiologic rise in pregnancy
- Obesity and metabolic syndrome
- Nephrotic syndrome
- Biliary obstruction
- Corticosteroid therapy
- Acute or chronic inflammation
- Acute infection
Elevated C3 most often reflects inflammation or metabolic disease and is not typically due to complement consumption.
References
- Lockitch G. Handbook of Diagnostic Biochemistry and Hematology in Normal Pregnancy. CRC Press; 1993.
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31.
- Wallach J. Interpretation of Diagnostic Tests. 8th ed. Lippincott Williams & Wilkins; 2007.