Objective <p>ART and pre-pregnancy BMI are strongly associated with neonatal outcomes, but there are fewer relevant studies. Our objective was to analyze the association between pre-pregnancy body mass index (BMI) and neonatal outcomes in women planning assisted reproductive technology (ART).</p> Methods <p>This was a retrospective study that included 994 ART singleton mothers who delivered in hospitals from July 2020 to June 2024. Mothers were categorized into 4 groups based on BMI (kg/m²): underweight (&lt; 18.5), normal weight (18.5–23.9, reference), overweight (24.0–27.9), and obesity (≥ 28.0). Associations between BMI classification and neonatal outcomes were analyzed using univariate and multivariate logistic regression.</p> Results <p>Among all mothers, pre-pregnancy underweight increased the adjusted odds ratio (aOR) for small for gestational age (SGA) to 3.97-fold compared to normal-weight mothers; overweight/obesity significantly increased the risk of macrosomia and large for gestational age (LGA), with aORs of 5.047 and 2.935-fold, respectively. In gestational weight gain (GWG) subgroup analyses, when GWG was adequate, pre-pregnancy underweight mothers were more likely to develop SGA than normal-weight mothers (aOR 4.649, 95% CI: 1.316–16.426), and overweight/obese mothers were not associated with adverse neonatal outcomes. When GWG was excessive, pre-pregnancy underweight mothers were 3.986 times more likely to be at risk of developing SGA than normal-weight mothers; pre-pregnancy overweight/obese mothers were 4.466 and 3.010 times more likely to develop macrosomia and LGA, respectively, compared to normal-weight mothers.</p> Conclusion <p>Maternal preconception underweight is associated with SGA regardless of whether GWG is adequate or excessive. For pre-pregnancy overweight or obese mothers, maintaining GWG in the target range can reduce the risk of adverse neonatal outcomes; if GWG is excessive, it is associated with macrosomia and LGA.</p>