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Protein C Activity — Pregnancy Reference Values

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Protein C activity (plasma)

Protein C is a vitamin K–dependent anticoagulant protein that inactivates Factors Va and VIIIa. During pregnancy, Protein C antigen levels remain largely stable, but activity may be mildly reduced due to increased consumption in the prothrombotic state of gestation.

Units Nonpregnant Adult 1st Trimester 2nd Trimester 3rd Trimester
% of normal 70 – 130 78 – 121 83 – 133 67 – 135
Pregnancy physiology
  • Total Protein C concentration remains relatively stable.
  • Protein C activity may be mildly reduced due to increased coagulation factor turnover.
  • Physiologic pregnancy is a prothrombotic state with reduced anticoagulant reserve.
  • True Protein C deficiency is uncommon and should prompt investigation.
Causes of low Protein C activity
  • Physiologic mild decrease in pregnancy
  • Hereditary Protein C deficiency (Type I or II)
  • Liver disease (reduced synthesis)
  • Vitamin K deficiency or malabsorption
  • Warfarin exposure
  • Disseminated intravascular coagulation (DIC)
  • Severe infection or sepsis
Causes of elevated Protein C activity
  • Compensatory increase after acute thrombosis
  • High-estrogen states (rare)
  • No common clinically significant pathologic causes
Clinical interpretation & pregnancy considerations
  • Isolated mild decreases are typically physiologic in pregnancy.
  • Markedly low activity suggests inherited deficiency, liver disease, or consumptive coagulopathy.
  • Hereditary Protein C deficiency increases risk for venous thromboembolism in pregnancy.
  • Testing should ideally be repeated postpartum for definitive diagnosis.
  • Interpret alongside Protein S, antithrombin III, and clinical thrombotic history.

References

  1. Abbassi-Ghanavati M, Greer LG, Cunningham FG. Pregnancy and laboratory studies: a reference table for clinicians. Obstet Gynecol. 2009;114:1326–31.
  2. Cunningham FG, Leveno KJ, Bloom SL, et al. Williams Obstetrics. 26th ed. Maternal coagulation physiology.
  3. Bates SM. Coagulation and thrombosis in pregnancy. Hematology Am Soc Hematol Educ Program. 2021.