Abstract: Objective To examine the association between maternal serum alanine aminotransferase (ALT) in the first trimester and the risk of macrosomia and large for gestational age(LGA)infant. Methods We used Tianjin Municipal Maternal and Child Health Information System and collected information on basic characteristics, antenatal care, detection result of ALT, and delivery outcome of 26 956 pregnant women with single-pregnancy who were registered for pregnancy and attended their first antenatal care within the first 12 weeks of gestation in 6 central urban districts and Binhai New District.Multivariate logistic regression was performed to obtain odds ratios (ORs) and 95% confidence intervals (CIs).To examine the joint effect of serum ALT and maternal obesity on the risks of macrosomia and LGA infant, the women were divided into four groups:non-obese group with ALT<40 U/L, obese and ALT<40 U/L, non-obese group with ALT≥40 U/L, and obese and ALT≥40 U/L. Results The median serum ALT at first antenatal care visit was 15.2 U/L (interquartile range:11.0-22.0 U/L) and 5.1% of the women showed the level of ALT≥40 U/L.Of all the newborns, 10.1% were macrosomia and 14.1% were LGA infants.The results of multivariate logistic regression demonstrated that the women with the ALT of ≥40 U/L were more likely to give birth to macrosomia (OR95% CI:1.1921.008-1.410) and LGA infant (1.1601.012-1.330) compared to those with the ALT of <40 U/L;the results also revealed that compared to the non-obese women with the ALT of <40 U/L, the women with obesity and/or increased serum ALT were at higher risk of giving birth to macrosomia (OR95% CIfor the obese with ALT <40 U/L:3.0112.593-3.496, for the non-obese with ALT ≥40 U/L:1.3201.097-1.589, and for the obese with ALT ≥40 U/L:3.8372.675-5.502) and to LGA infant (OR95% CIfor the obese with ALT <40 U/L:2.7812.458-3.147, for the non-obese with ALT ≥40 U/L:1.2381.066-1.437, and for the obese with ALT ≥40 U/L:3.7662.757-5.144). Conclusion Elevated ALT in first trimester alone or combined with maternal obesity correlate with the increased risk of giving birth to macrosomia and large for gestational age infant.