Changes in protein C and protein S levels in normal pregnancy

OBJECTIVE: The objective of the study was to determine the normal changes in the plasma concentrations of protein C and protein S that occur during each trimester of pregnancy. STUDY DESIGN: The study was a prospective cross-sectional study of 91 normal pregnant women who had plasma concentrations of protein C and protein S measured during the first, second, and third trimesters. RESULTS: There was no statistically significant change in antigenic or functional protein C levels during normal pregnacy. Total protein S levels also remained unchanged. Free protein S levels fell significantly from first to second trimesters (0.45 U/ml mean to 0.26 U/ml mean, p< 0.001), but no further fall occurred during the third trimester. CONCLUSIONS: The second-trimester fall in free protein S levels is a physiologic pregnancy adaptation. Women with a thromboembolic event appearing for the first time during pregnancy should have investigations for protein S deficiency delayed until the postpartum period, to avoid misdiagnosis and treatment