Background: Immature granulocyte (IG) counts as a powerful clinical parameter can be obtained by new generations of hematology analyzers automatically. Establishment of IG reference intervals in healthy pregnant women is a prerequisite for clinically meaningful interpretation of the parameter. Methods: By using a Sysmex XE-5000 hematology analyzer, a large number of pregnant women comprising 1372 samples were analyzed to determine pregnancy trimester stratified reference intervals for IG, including their absolute values (#IG) and relative values (%IG). Results: Compared with healthy non-pregnant women, #IG and %IG in healthy pregnant women increased significantly (P<0.05). Although there was no difference in #IG and %IG of healthy pregnant women of different ages, these values were positively correlated with pregnancy trimester and there was significant difference among the first, second, and third trimester groups (P<0.05). According to a non-parametric statistical method, the reference intervals for #IG in the first, second, and third trimesters were (0.003-0.091)×109/L, (0.007-0.247)×109/L, and (0.018-0.456)×109/L, respectively, while those for %IG were (0.04-0.92)%, (0.10-2.00)%, and (0.20-3.80)%, respectively. Conclusion: This study demonstrates IG counts in healthy pregnant women increase significantly during pregnancy, especially in the second and third trimesters. IG reference intervals according to pregnancy trimester have been established, and it may be used for evaluating health status of pregnant women and help obstetricians to make their clinical decisions.