PurposeWe aimed to investigate whether the negative effects of maternal depressive symptoms on Small for gestational age (SGA) was moderated by pre-pregnancy body mass index.MethodsA total of 9,851 women provided self-reported height and pre-pregnancy weight at their first prenatal visit. Pre-pregnancy BMI was categorized according to the BMI cut point ranges for Chinese adults. Depression during pregnancy was assessed by using standardized questionnaires. Demographic characteristics and birth outcomes, including gestational age at birth and birth weight, were collected through the interviews and case history. Multiple regression analysis was used to estimate the relationship between depressive symptoms and SGA among various pre-pregnancy BMI groups.ResultsThe relationship between depressive symptoms and SGA was inconsistent among three pre-pregnancy BMI groups. The incidence of SGA was positively related with depressive symptoms only among women with pre-pregnancy underweight [odds ratio (OR) 1.89, 95 % confidence interval (CI) 1.12–3.21].ConclusionsThe negative effect of depressive symptoms during pregnancy on SGA depends on pre-pregnancy BMI.