摘要Background Birth weight is a strong determinant of infant short-and long-term health outcomes.Family socioeconomic position(SEP)is usually positively associated with birth weight.Whether this association extends to abnormal birth weight or there exists potential mediator is unclear.Methods We analyzed data from 14,984 mother-infant dyads from the Born in Guangzhou Cohort Study.We used multi-variable logistic regression to assess the associations of a composite family SEP score quartile with macrosomia and low birth weight(LBW),and examined the potential mediation effect of maternal pre-pregnancy body mass index(BMI)using causal mediation analysis.Results The prevalence of macrosomia and LBW was 2.62％(n=392)and 4.26％(n=638).Higher family SEP was asso-ciated with a higher risk of macrosomia(OR 1.30,95％CI 0.93-1.82;OR 1.53,95％CI 1.11-2.11;and OR 1.59,95％CI 1.15-2.20 for the 2nd,3rd,and 4th SEP quartile respectively)and a lower risk of LBW(OR 0.69,95％CI 0.55-0.86;OR 0.76,95％CI 0.61-0.94;and OR 0.61,95％CI 0.48-0.77 for the 2nd,3rd,and 4th SEP quartile respectively),compared to the 1st SEP quartile.We found that pre-pregnancy BMI did not mediate the associations of SEP with macrosomia and LBW.Conclusions Socioeconomic disparities in fetal macrosomia and LBW exist in Southern China.Whether the results can be applied to other populations should be further investigated.