SESSION TITLE: Tobacco Cessation and Prevention Posters SESSION TYPE: Original Investigation Posters PRESENTED ON: October 18-21, 2020 PURPOSE: Research has shown smoking and smoking cessation are associated with inadequate and excessive gestational weight gain (GWG), respectively. There is poor knowledge on critical timings (weeks of pregnancy) of abnormal GWG among smokers and ex-smokers, which can inform weight management intervention. Also, the long-term postpartum consequences of abnormal GWG have been understudied among smokers and ex-smokers specifically. We aimed to fill these research gaps by examining their weight trajectories during pregnancy and postpartum. METHODS: In the UB Pregnancy and Smoking Cessation Study, 56 pregnant smokers were recruited from 2015-2019. Expectant mothers attended numerous visits throughout pregnancy and postpartum, where smoking status was verified and repeated weight measurements were taken. We calculated the pre-pregnancy BMI and total GWG for each mother. Using the Institute of Medicine (IOM) guidelines, the total GWG of each mother is classified into 3 categories (inadequate, appropriate, and excessive) according to pre-pregnancy BMI. GWG Trajectories were analyzed by fitting a linear regression model to a polynomial function and evaluating the interaction terms between smoking status and polynomial functions of gestational age. Next, we calculated Postpartum Weight Retention (PPWR) for each mother. We categorized mothers with substantial PPWR (≥ 5 kg (11 lbs.) at one year after delivery. Postpartum weight trajectories were divided into two phases (from delivery-2 weeks, and from 2 weeks- 12 months) and then analyzed similarly to GWG for each phase. The quit date is examined as a critical pregnancy time-point in the fate of GWG and PPWR through comparison of the GWG trajectories of early and late quitters. RESULTS: Quitters and non-quitters had similar GWG trajectories in the first trimester, but differed substantially afterwards: quitters increasing rapidly vs. non-quitters plateauing then stabilizing. As a result, 65% of quitters had excessive total GWG, and 60% of non-quitters had inadequate total GWG (p-value=0.001) by the end of pregnancy. Mothers who quit early in pregnancy were at much higher risk of excessive GWG than those who quit late in pregnancy. In postpartum, quitters and non-quitters have parallel weight trajectories, however, Quitters have higher PPWR at all time points. Quitters had a higher mean PPWR (12.9 lbs.) than non-quitters (-3.1 lbs.) by 12 months postpartum. CONCLUSIONS: Quitters and non-quitters differed in weight trajectories after the first trimester. Quitting smoking during pregnancy can lead to excessive GWG and substantial PPWR, especially among early-pregnancy quitters. CLINICAL IMPLICATIONS: The data gathered important to analyze because the recommendations for healthy GWG include both smokers and non-smokers. Smoking mothers may fit into a special category that may require the establishment of a new set of healthy GWG trajectories. DISCLOSURES: No relevant relationships by Paul Mercado, source=Web Response no disclosure on file for Joshua Sorrentino; No relevant relationships by Xiaozhong Wen, source=Web Response