Objectives Gestational weight gain (GWG) is associated with fetal and newborn health; however, data from sub-Saharan Africa are limited. We estimated the relationship between GWG and newborn outcomes in a cohort of pregnant women in Tanzania. Methods We used data from a prenatal multiple micronutrient supplementation trial among HIV-negative pregnant women enrolled between 12 to 27 weeks gestational age (GA) in Dar es Salaam. GWG adequacy was defined as the ratio of the total observed weight

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... n over the expected weight gain based on the Institute of Medicine pre-pregnancy body mass index (BMI)-specific guidelines, and categorized as follows: severely inadequate (< 70%), inadequate (70–90%), adequate (90–125%) or excessive (≥125%). Newborn outcomes assessed were: stillbirth, perinatal death, preterm birth (birth < 37 weeks GA), low birthweight (LBW; < 2500 grams), macrosomia (birthweight > 4000 grams), small- and large-for-GA (birthweight-for-GA < 10th percentile and > 90th percentile, respectively), short-for-GA (length-for-GA z-score < -2SD), and microcephaly (head circumference-for-GA z-score < -2SD). Modified Poisson regression with robust standard error was used to estimate the relative risk of newborn outcomes as a function of GWG adequacy, adjusting for confounders. Results Overall, 51% (n = 3866/7561) women had severely inadequate or inadequate GWG, 31% had adequate GWG, and 18% had excessive GWG by delivery. Compared to adequate GWG, severely inadequate GWG was associated with increased risk of LBW [Adjusted Relative Risk (95% CI): 1.63 (1.24, 2.14)], SGA [1.76 (1.50, 2.08)], short-for-GA [1.24 (1.04, 1.48)], and microcephaly [1.35 (1.05, 1.73)]; but a lower risk of LGA [0.72 (0.58, 0.89)] and macrosomia [0.31 (0.17, 0.56)]. Inadequate GWG was similarly associated with a higher risk of SGA, short-for-GA, and microcephaly, but lower risk of LGA. Excessive GWG was only associated with a higher risk of LGA and macrosomia. Severely inadequate GWG in the second trimester was associated with preterm birth. GWG adequacy was not associated with stillbirth or perinatal death. Conclusions Inadequate GWG was associated with several indicators of poor newborn size. Interventions to support maternal nutrition during pregnancy are needed and are likely to improve newborn outcomes. Funding Sources Bill and Melinda Gates Foundation; CIHR Fellowship.