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Excessive gestational weight gain contributes to adverse maternal and neonatal outcomes. Environmental exposures such as phthalates may lead to metabolic dysregulation, and studies suggest possible associations between maternal phthalate exposure and altered gestational weight gain. We assessed the association between nine maternal phthalate metabolites and measures of total gestational weight gain (pre-pregnancy to median 35.1 weeks of gestation) in a case-control study nested within LIFECODES (N = 379), a prospective birth cohort from Boston, Massachusetts (2006–2008). Our primary outcome was total gestational weight gain z score, a measure independent of gestational age that can provide a less biased estimate of this association. Our secondary outcomes were total gestational weight gain, rate of gestational weight gain, and adequacy ratio. The results were stratified by pre-pregnancy body mass index category. We found that concentrations of mono-(3-carboxypropyl) phthalate (MCPP) and mono-n-butyl phthalate (MBP) were positively associated with total gestational weight gain z scores among participants with obesity: adjusted mean difference (95% Confidence Interval [CI]) = 0.242 (0.030 – 0.455) and 0.105 (-0.002 – 0.212) corresponding to an excess weight gain of 1.81 kg and 0.77 kg at 35 weeks of gestation per interquartile range-increase in MCPP and MBP, respectively. Also, among participants with obesity, MBP demonstrated a potential non-linear relationship with gestational weight gain in cubic spline models. These findings suggest that phthalates may be related to higher gestational weight gain, specifically, among individuals with pre-pregnancy obesity. Future research should investigate whether pregnant people with obesity represent a subpopulation with sensitivity to phthalate exposures. • Associations between phthalates and gestational weight gain differed by pre-pregnancy BMI. • MCPP and MBP were associated with higher gestational weight gain among participants with obesity. • Some phthalates, including MBP, had non-linear associations with gestational weight gain. [ABSTRACT FROM AUTHOR]