Objectives Hypertensive disorders of pregnancy (HDP) is a pregnancy complication that increases the risk of preterm delivery and cesarean delivery. This study aimed to investigate whether maternal blood monocyte or neutrophil counts in the first trimester are related to the development of HDP. Design, Setting and Sample Data were collected from the Japan Environment and Children’s Study, a large birth cohort study (n = 38194) that recruited pregnant women in 15 Regional Centers across Japan between January 2011 and March 2014. The maternal neutrophil and monocyte counts in the first trimester were divided into quartiles. Results Compared with pregnant women with low neutrophil counts, those with high neutrophil counts had higher adjusted odds ratio (aOR) for both mild HDP (aOR [95% confidence interval]: Q4: 1.29 [1.06–1.58]) and severe HDP (aOR [95% CI]; Q2: 1.49 [1.07–2.08], Q3: 1.40 [1.00–1.95], and Q4: 1.47 [1.06–2.03]). Furthermore, compared with pregnant women with lower monocyte counts, those with higher monocyte counts also had higher aOR for both moderate HDP (aOR [95% CI]; Q2: 1.28 [1.02–1.60], Q3: 1.42 [1.14–1.77], and Q4: 1.49 [1.20–1.85]) and severe HDP (aOR [95% CI]; Q2: 1.33 [1.06–1.66], Q3: 1.46 [1.17–1.82], and Q4: 1.52 [1.22–1.89]). Conclusion In conclusion, high leukocyte counts, especially high monocyte count, in the first trimester is associated with a development of HDP. Thus, they may be used to predict subsequent HDP.