- Purpose: To examine whether an association exists between maternal leukocyte count in the first trimester of pregnancy and the risk for development of obstetric complications. Methods: The study population included all the registered births (n = 33,866) with available white blood cell count (WBC) from the first trimester of pregnancy, during 2000-2010 in a tertiary medical center. The leukocyte count was sorted by the following groups: WBC < 3.5 10 × 9/l, WBC = 3.5 10 × 9/l-13.8 10 × 9/l, and WBC > 13.8 10 × 9/l. These groups were compared regarding obstetrical characteristics and perinatal outcomes. Multiple logistic regression models were used to control for confounders. Results: The study population included 33,866 deliveries, of these 0.94 % had leukocytosis WBC > 13.8 10 × 9/l. Women with leukocytosis delivered significantly earlier, were more likely to be Jewish, and in their first pregnancy. There were significantly higher rates of preterm delivery before 37 weeks, fertility treatments, hypertensive disorders, gestational diabetes mellitus, and cesarean section among these patients. There was no significant difference between the groups regarding preterm premature rupture of membranes (PPROM) and chorioamnionitis. Women with leukocytosis during the first trimester had significantly higher rates of fetuses who were small for gestational age and with birth weight <2,500 g. Multivariable analysis showed a significant association between preterm delivery before 37 weeks and leukocytosis during the first trimester of pregnancy (p = 0.003). Conclusions: Leukocytosis during the first trimester is significantly associated with an increased risk for obstetrical complications, in particular preterm delivery. Nevertheless, further studies should focus on women with leukocytosis during the first trimester in order to verify our results.