摘要Background Despite extensive research evaluating the association between prenatal exposure to secondhand smoke(SHS)and the development of autism spectrum disorders(ASD),no study has investigated the association by considering the pre-conceptional period.This study aimed to investigate the associations of pre-conceptional and prenatal SHS exposure and the development of ASD among toddlers.Methods In this cross-sectional study,parents of 6049 toddlers aged 16-30 months were recruited from 7 tertiary hospi-tals,21 communities,and 7 kindergartens located in seven cities in six provinces from five geographical regions of China.We analyzed the associations of SHS exposure and the odds of ASD among toddlers in different exposure windows(pre-conceptional and/or prenatal periods).Data were analyzed from November 2021 to January 2022.Results Among the 6049 toddlers included in the analysis[22.7(4.1)months;44.8％girls],71 were identified and diagnosed with ASD.Compared with the unexposed toddlers,toddlers with pre-conceptional SHS exposure had higher odds of ASD(OR 2.30,95％CI 1.36-3.84),while we observed a non-significantly positive association regarding prenatal SHS exposure.When considering both pre-conceptional and prenatal periods,toddlers who were continuously exposed to SHS during these two periods had higher odds than those without SHS exposure(OR 2.32,95％CI 1.24-4.14).Conclusion We reported positive SHS-ASD associations when exposed during the pre-conceptional period and continu-ously exposed during pre-conceptional and prenatal periods,emphasizing the critical window of pre-conception for targeted intervention on smoking.