Pregnancy is associated with significant haemostatic changes, with a progressive rise in most clotting factors. There is limited data on the changes of factor XIII (FXIII) level during pregnancy. This study assesses changes in FXIII activity during normal pregnancy and establish FXIII reference range during each trimester of pregnancy and immediate postnatal period. This is a cross sectional study of 376 women with normal uneventful pregnancies. Plasma FXIII activity was measured during first (weeks 0-12, n = 116), second (weeks 13-28, n = 132), third trimester (weeks 29-42, n = 128) and postnatal (day 0-3; n = 30). Samples were also collected from non-pregnant women (n = 25) as a control group. FXIII was assayed on CS-5100 analyser using chromogenic reagent. The mean ± SD FXIII activity was 112 ± 29 IU dL(-1) during first trimester, 96 ± 26 IU dL(-1) during second trimester, 83 ± 21 IU dL(-1) during third trimester, 90 ± 19 IU dL(-1) during postnatal period, and 113 ± 26 IU dL(-1) in the control. The reference range was calculated during the first (55-169 IU dL(-1)), second (45-147 IU dL(-1)), third trimester (42-125 IU dL(-1)) and postnatal period (61-137 IU dL(-1)). There was a significant reduction in the mean FXIII activity during the second and third trimester compared to the first trimester and control group (P < 0.0001). During the immediate postnatal period, the mean FXIII activity was not statistically different compared to the third and second trimester levels but was significantly lower compared to the first trimester (P < 0.0001) level and the control group (P = 0.0002). This study establishes the reference range for FXIII activity during the three trimesters of normal pregnancy and immediate postnatal period. Women have a significantly decreased level of FXIII activity during a normal uneventful pregnancy.