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Introduction: Since 2013, various guidelines for hypertension in pregnancy have been refined, no longer requiring proteinuria as a requisite criterion for preeclampsia. We aimed to evaluate the impact of the new definition on preterm birth (PTB) and adverse pregnancy outcomes., Material and Methods: Women delivering in Ontario between April 2012 and November 2016 were included. Delivery <24+0/7 weeks, major fetal anomalies or preexisting renal disease were excluded. The primary outcome was livebirth <37, <34 or <32 weeks. Rates, adjusted rate ratios (aRR) and ratio of the rate ratio (RRR) were used to compare outcomes in the 2 years after the new Society of Obstetricians and Gynaecologists of Canada (SOGC) guideline (December 2014-November 2016; period 2) vs the 2 years before (April 2012-March 2014; period 1), among women with and without preeclampsia., Results: In all, 268 543 and 267 964 births in periods 1 & 2, respectively, were included. Respective preeclampsia rates increased significantly from 3.9% to 4.4% (p < 0.001), with no change in maternal morbidity rates. In preeclamptic women, respective rates of PTB <37 weeks were 21.0% and 20.7% (aRR 1.01, 95% confidence interval [CI] 1.00-1.02), with significant aRR for PTB <34 (0.86, 95% CI 0.77-0.96) and <32 weeks (0.79, 95% CI 0.67-0.94). A similar aRR was observed in women without preeclampsia. In preeclamptic women, composite severe neonatal morbidity decreased after guideline change (aRR 0.95, 95% CI 0.91-0.99), a finding not observed in women without preeclampsia (RRR 0.95, 95% CI 0.91-0.99)., Conclusions: The new definition of preeclampsia was associated with increased disease rates, a modest reduction in adverse neonatal outcomes and no change in maternal outcomes., (© 2021 Nordic Federation of Societies of Obstetrics and Gynecology (NFOG). Published by John Wiley & Sons Ltd.)