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Background/Aims: Phthalates are endocrine disruptors with ubiquitous exposure in pregnant women, but their impact on maternal gestational weight gain (GWG) is unclear. We investigated associations of prenatal exposure with GWG and considered whether associations differed by maternal pre-pregnancy BMI (ppBMI). Methods: In 444 pregnant women from Champaign-Urbana, Illinois, we calculated GWG (kg) from self-reported pre-pregnancy weight and weight at median (range) 34 (29-38) weeks gestation. We quantified 19 phthalate/alternative metabolites in pools of five first-morning cross-pregnancy urines. These reflect exposures to 10 phthalates/alternatives as individual metabolites or molar sums of metabolites from the same phthalate/alternative [e.g. di(2-ethylhexyl) phthalate (SumDEHP), diisononyl phthalate (SumDiNP), di(2-ethylhexyl) terephthalate (SumDEHTP)]. We categorized ppBMI (kg/m²) as under-/normal weight (<25.0), overweight (25.0-29.9), and obese (≥30.0). We used linear regression to assess overall and ppBMI-stratified associations of ln-transformed specific gravity-adjusted biomarker concentrations with GWG, controlling for maternal age, race, ppBMI, parity, smoking status, diet quality, fetal sex, and gestational age at mid-to-late pregnancy weight. Results: These predominately Caucasian/White, college-educated women gained on average 12.1kg through mid-to-late pregnancy and >50% had a normal ppBMI. All women were exposed to measured chemicals, with similar concentrations to U.S. women. Overall, only SumDEHP and SumDEHTP were associated with GWG. Specifically, two-fold increases in SumDEHP and SumDEHTP were associated with 0.8kg lower GWG (95%CI: -1.5, -0.7 and -1.4, -0.2), respectively. However, when stratified by ppBMI, associations were strongest in obese women. Only in obese women, two-fold increases in SumDEHP and SumDiNP were associated with 1.9 and 1.3kg lower GWG, respectively. Similarly, in overweight and obese women, a two-fold increase in SumDEHTP was associated with 1.2 and 1.6kg lower GWG. Conclusions: In obese women, prenatal exposure to phthalates is associated with reduced GWG. Given the importance of appropriate GWG for fetal growth, especially in obese women, these findings should be corroborated in other populations.