Table 2. Associations between maternal preconception serum ALT concentrations and the risk of PTB among 5,169,812 reproductive-aged women in China, 2013–2017.

ALT level (U/L)	No. cases (%)	OR (95% CI)
		Model Ⅰ*	Model Ⅱ†	Model Ⅲ§
Abbreviation: PTB=preterm birth; ALT=alanine aminotransferase; OR=odds ratio; CI=confidence interval. * Model Ⅰ was an unadjusted model. † Model Ⅱ was adjusted for maternal age at the last menstrual period, preconception body mass index (BMI), education, nationality, occupation, residence, region, parity, hypertension, hyperglycemia, smoking, drinking, periconception folic acid intake, harmful exposure, and history of adverse pregnancy outcomes. § Model Ⅲ was additionally adjusted for maternal serum hepatitis B surface antigen (HBsAg) status.
Classification Ⅰ
≤40	324,465 (6.61)	1.00 (Reference)	1.00 (Reference)	1.00 (Reference)
>40	19,527 (7.38)	1.12 (1.11, 1.14)	1.11 (1.09, 1.13)	1.11 (1.09, 1.12)
Classification Ⅱ
<20	218,403 (6.78)	1.08 (1.07, 1.09)	1.06 (1.06, 1.07)	1.07 (1.06, 1.07)
20–40	106,062 (6.31)	1.00 (Reference)	1.00 (Reference)	1.00 (Reference)
>40	19,527 (7.38)	1.18 (1.16, 1.20)	1.16 (1.14, 1.17)	1.15 (1.13, 1.17)