Abstract Background It is unclear whether the effects of abnormal gestational weight gain (GWG) on birth outcomes are differently in women with different maternal ages. This study aimed to investigate maternal age-specific association between GWG and adverse birth weights in Chinese women older than 30. Methods 19,854 mother-child dyads were selected from a prospective cohort study in Southwest China between 2019 and 2022. Logistic regression model was used to assess the association between GWG, which defined by the 2009 Institute of Medicine guidelines, and adverse birth weights including large- and small-for-gestational-age (LGA and SGA), stratified by maternal age (31–34 years and ≥ 35 years). Results In both maternal age groups, excessive and insufficient GWG were associated with increased odds of LGA and SGA, respectively. After women were categorized by pre-pregnancy body mass index, the associations remained significant in women aged 31–34 years, whereas for women aged ≥ 35 years, the association between excessive GWG and the risk of LGA was only significant in normal weight and overweight/obese women, and the significant effect of insufficient GWG on the risk of SGA was only observed in underweight and overweight/obese women. Moreover, among overweight/obese women, the magnitude of the association between insufficient GWG and the risk of SGA was greater in those aged ≥ 35 years (31–34 years: OR 2.08, 95% CI 1.19–3.55; ≥35 years: OR 2.65, 95% CI 1.47–4.74), while the impact of excessive GWG on the risk of LGA was more pronounced in those aged 31–34 years (31–34 years: OR 2.18, 95% CI 1.68–2.88; ≥35 years: OR 1.71, 95% CI 1.30–2.25). Conclusions The stronger associations between abnormal GWG and adverse birth weights were mainly observed in women aged 31–34 years, and more attention should be paid to this age group.
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