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 multivariable 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 associated 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.