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426 Defining the gestational age cut-off between early and late preeclampsia in singletons Amir Aviram, Jon Barrett, Arthur Zaltz, Christopher Sherman, John Kingdon, Nir Melamed Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada, University of Toronto, Toronto, Ontario, Canada, Mount Sinai Hospital, Toronto, Ontario, Canada OBJECTIVE: It has been suggested that early onset preeclampsia (EPE) and late onset preeclampsia (LPE) represent two different and distinct entities. LPE is more common, and usually less severe than EPE. Yet, while these terms are widely used, there is no clear definition of the gestational age cut-off between early and late preeclampsia, and various studies named different thresholds between 32 and 36 weeks. We aimed to establish this gestational age cut-off using placental pathology, mainly maternal vascular malperfusion lesions (MVM’s), which are the hallmark of placental findings in preeclampsia. STUDY DESIGN: This was a retrospective analysis of all women with singleton gestations, at or beyond 24 weeks of gestation, diagnosed with preeclampsia, who delivered between 2001 and 2015 at a single, tertiary referral center. Placental abnormalities were classified into lesions related to maternal vascular malperfusion, fetal vascular malperfusion, lesions associated with hemorrhage and chronic inflammation. Placental findings were compared by gestational age at delivery. RESULTS: A total of 430 women with singleton gestation and preeclampsia were eligible for analysis. Out of all of the placental pathology parameters examined, MVM’s emerged as potential candidate to define the gestational age cut-off between LPE and EPE, as the prevalence of these findings decreased significantly at 35 weeks of gestation (Figure). The prevalence of one or more MVM’s was significantly higher < 35 weeks of gestation (93.2% vs. 47.3%, p<0.001) (Table). Comparing the prevalence of 2 and 3 different types of MVM’s (as more sensitive markers of placental ischemia), before and after 35 weeks of gestation, the same pattern was even more pronounced, (68.2% vs. 13.3%, p<0.001 for 2 MVM’s and 33.2% vs. 3.3%, p<0.001 for 3 MVM’s). In a multivariable regression model, maternal age and nulliparity were not found to influence this association. In a sub-analysis of only appropriately grown neonates (birthweight>10% percentile, n1⁄4324), 35 gestational weeks remained a valid threshold (91.6% vs. 37.2% for 1 MVM’s, 61.6% vs. 6.6% for 2 MVM’s and 28.1% vs. 1.7% for 3 MVM’s, p<0.001 for all) (Table) CONCLUSION: Based on the differences in the prevalence of MVM’s in placental pathology specimens, the gestational age cut-off between early and late preeclampsia is 35 weeks.