OBJECTIVE: To investigate the relationship between platelet indices (count, size and production/immaturity) and hypertensive disorders of pregnancy. STUDY DESIGN: This was a secondary analysis of a prospective cohort of pregnant individuals followed from first trimester through delivery at an academic tertiary care institution. Routine platelet indices obtained prospectively during prenatal care and delivery were compared between those who developed a hypertensive disorder of pregnancy and those who did not. We assessed platelet count (by trimester), mean platelet volume, and immature platelet fraction measured as percent (%) and absolute count. Data were analyzed using Fisher's Exact test, chi-square test, and multivariable logistic regression. P <
0.05 was considered statistically significant. RESULTS: Among 459 participants enrolled, 78 (17.0 %) developed a hypertensive disorder of pregnancy. Pregnant individuals who developed a hypertensive disorder of pregnancy had significantly higher mean platelet volume (11.7 fL [10.8,12.4] vs. 11.1 fL [10.5,12.0], P CONCLUSIONS: An increase in platelet size and immaturity was observed in those with a hypertensive disorder of pregnancy. These data support further investigation of platelets in the mechanisms of the development of hypertensive disorders of pregnancy and the use of platelet indices to better identify high risk groups in pregnancy.