Reference range for C1-esterase inhibitor (C1 INH) in the third trimester of pregnancy

Objectives The objectives of this study were to (i) establish the reference range and mean value for normal levels of C1-esterase inhibitor (C1 INH) during pregnancy, and (ii) investigate the association between C1 INH and uterine atony, as measured by blood loss at delivery. Methods We prospectively studied 200 healthy pregnant women who were registered. We studied C1 INH levels in 188 women at 34 and 35 gestational weeks of pregnancy. The reference range for C1 INH during the third trimester of pregnancy was calculated using the value of C1 INH that was determined at registration. Results The mean value of C1 INH was determined to be 70.3% (95% confidence interval, 68.7–71.9). While the C1 INH levels in four women were determined to be 40% lower than the calculated mean value, amniotic fluid embolism (AFE) did not occur in any of the women studied. Conclusions This study successfully demonstrated that a reference value for C1 INH activity can be established using the methods described herein. Further research is needed to determine whether C1 INH is involved in obstetric coagulopathy syndrome such as amniotic fluid embolism.