Objective: To estimate the risk of preterm preeclampsia in primiparous women by pre-pregnancy obesity class. Methods: A retrospective cohort study of primiparous women with singleton gestations was performed for deliveries from January 2003 to April 2014. Cases were stratified by delivery occurring either at ≥ 37 weeks or < 37 weeks. Pre-pregnancy maternal obesity was defined as a body mass index (BMI) ≥ 30 kg/m2. World Health Organization criteria were used to define BMI class of obesity. Multinomial logistic regression modeling estimated the association between term and preterm preeclampsia and pre-pregnancy obesity. Results: Of 28,361 women with complete pre-pregnancy BMI data, 2,588 women (9.1%) had a diagnosis of preeclampsia. Women who developed preeclampsia prior to 37 weeks (n = 784) were more likely to be women with obesity compared to women who developed preeclampsia after 37 weeks (33.1% vs. 25.3%, P = 0.0001). Compared to normal-weight women without preeclampsia, the risk of preterm preeclampsia increased proportionally with pre-pregnancy obesity class, with women with a BMI ≥ 40 kg/m2 having the greatest risk (RR 5.23, 95% CI: 3.86-7.09, P <0.001). Conclusions: The risk of preterm preeclampsia increased significantly as the severity of maternal pre-pregnancy obesity increased. Reduction in maternal pre-pregnancy BMI may be protective in mitigating such risk.