Preplanned Studies: Maternal Preconception Serum Alanine Aminotransferase Levels and Risk of Preterm Birth among Reproductive-Aged Women — China, 2013–2017

Preterm birth (PTB), defined as the delivery of an infant prior to 37 complete weeks of gestation, accounted for approximately 10.9% of all live births worldwide in 2019 and has emerged as the leading cause of neonatal and under-five mortality (1–3). Recently, maternal liver health has become a focus of interest, given its potential influence on PTB (4–6). Serum alanine aminotransferase (ALT), a critical biomarker for evaluating hepatocellular injury and liver dysfunction (7), suggests that regular monitoring and maintaining ALT levels within an appropriate range before and during pregnancy may enhance maternal health and decrease the likelihood of PTB. Current clinical references for ALT, however, are derived from a generalized population pool consisting of both sexes, raising the question of their validity for preconception women, a concern that lacks empirical support. In this study, we examined the link between preconception maternal serum ALT concentrations and PTB risk within a cohort of reproductive-aged Chinese women. The findings revealed that abnormal increases, as well as somewhat diminished levels, of preconception serum ALT are associated with PTB and its subcategories, presenting a non-linear, J-shaped correlation.

This retrospective cohort study was carried out by the National Free Preconception Checkups Project (NFPCP), a nationwide initiative offering complimentary preconception health services, including examinations, counseling, and post-conception outcome monitoring to couples of reproductive age aiming to start a family. Further specifics regarding the structure, management, and execution of the NFPCP have been documented in previous publications (8–10).

Between January 1, 2013, and December 31, 2016, a total of 5,817,003 Chinese women aged 20 to 49 who took part in the NFPCP successfully conceived within a year after their preconception examination. By December 31, 2017, all participants were followed up on their pregnancy outcomes. Exclusions were made for those with a history of multiparous pregnancies (n=433,212), non-viable births (such as fetal death, ectopic pregnancy, spontaneous abortion, included therapeutic or medical abortions) (n=21,720), or self-reported medication use (n=174,835). Individuals with missing preconception serum ALT values (n=17,414) or insufficient information on gestational weeks (n=10) were also removed from the study.

Experienced doctors conducted physical examinations and routine clinical laboratory measurements following standard guidelines. Venous blood samples, collected after an 8-hour fasting period, were stored at −30 °C and analyzed within 24 hours for ALT, blood glucose, and hepatitis B surface antigen (HBsAg). Serum ALT levels were analyzed using an automatic biochemical analyzer by measuring the absorbance change at 340 nm following a substrate reaction with the serum.

The gestational age was determined by the interval in weeks between the date of the last menstrual period (LMP) adjusted by ultrasound examination and the delivery date. PTB was defined as live birth before completion of 37 weeks of gestation, with further classification into moderate to late preterm birth (MPTB, 32 to less than 37 weeks), very preterm birth (VPTB, 28 to less than 32 weeks), and extremely preterm birth (EPTB, less than 28 weeks) based on gestational age (1).

Baseline characteristics were presented as counts and percentages, means with standard deviations (SDs), or medians with interquartile ranges (IQRs). Group differences in ALT levels were analyzed using the χ² test, Fisher’s exact test, or Kruskal-Wallis H test. The relationship between maternal preconception serum ALT levels and PTB was examined through restricted cubic spline (RCS) curves, with nonlinearities assessed using Wald statistics. ALT levels were categorized based on general thresholds (normal: ≤40 U/L; elevated: >40 U/L) and population-specific threshold ranges derived from RCS curves: <20 U/L, 20–40 U/L (reference range), and >40 U/L.

Logistic regression models were utilized to calculate odds ratios (ORs) and 95% confidence intervals (CIs), with and without adjusting for pre-selected covariates, to assess the relationship between maternal preconception serum ALT levels and the risk of PTB and its subtypes. The covariates used for adjustment are described in Supplementary Table S1. Subgroup analyses were conducted to confirm the consistency of the association across different populations after controlling for multiple covariates. Statistical analyses were carried out using R software (version 4.2.2; R Foundation for Statistical Computing, Vienna, Austria). All tests were two-sided, and a P-value <0.05 was considered statistically significant.

A total of 5,169,812 women were included in this cohort study, with an average age of 26.52 years (SD: 4.21) and a PTB incidence rate of 6.65% (343,992 events) (Figure 1). PTB rates across different serum ALT level groups — <20 U/L, 20–40 U/L, and >40 U/L — were 6.78% (218,403 events), 6.31% (106,062 events), and 7.38% (19,527 events), respectively, with the lowest rate observed in the 20–40 U/L group. Women with preconception ALT levels <20 U/L had a higher likelihood of being underweight [body mass index (BMI) <18.5 kg/m², n (%): 481,090 (14.92%) vs. 189,707 (11.28%)], whereas those with ALT levels >40 U/L were more inclined to be overweight or obese [BMI of 24.0–27.9 kg/m²: 46,612 (17.61%) vs. 232,028 (13.80%); BMI ≥28 kg/m²: 16,438 (6.21%) vs. 57,561 (3.42%)] (P<0.001) (Table 1).

Figure 1. 

Flowchart illustrating the study population of reproductive-aged women in China from 2013 to 2017.

The dose-response relationship between preconception serum ALT levels in mothers and various categories of PTB is delineated in Figure 2, which suggests a J-shaped association. Statistically significant nonlinear effects were observed for all categories of PTB, including MPTB, VPTB, and EPTB, with chi-square values and P-values indicating nonlinearity as follows: for PTB (χ²=613.12, P<0.001), MPTB (χ²=476.43, P<0.001), VPTB (χ²=128.03, P<0.001), and EPTB (χ²=138.01, P<0.001). Notably, critical serum ALT threshold ranges were identified: 17 to 40 U/L for PTB and MPTB, 17 to 37 U/L for VPTB, and 15 to 31 U/L for EPTB. Compared to the reference group with preconception ALT levels between 20–40 U/L, women with ALT levels below 20 U/L had a 7% increased risk of PTB (OR: 1.07, 95% CI: 1.06, 1.07), while those with levels above 40 U/L had a 15% higher risk (OR: 1.15, 95% CI: 1.13, 1.17), as outlined in Table 2. Similar patterns were evident across different PTB subtypes (Table 3). Subgroup analyses revealed that the observed associations were consistent across most subgroups and were not markedly affected by modifying factors. Compared to women with preconception ALT levels within the reference range, both lower and higher ALT concentrations were linked to increased PTB risk in the majority of subgroups, as illustrated in Figure 3.

Figure 2. 

Restricted cubic spline curves with logistic regression between maternal preconception serum ALT concentrations and risk of (A) PTB (<37 weeks), (B) MPTB (32 to <37 weeks), (C) VPTB (28 to <32 weeks), and (D) EPTB (<28 weeks) among 5,169,812 reproductive-aged women in China, 2013–2017.

Note: Red dashed horizontal lines depict an odds ratio of 1.0. Red lines represent the estimated OR, while shaded ribbons depict a 95% CI. The dashed line denotes the reference level. The models were adjusted for maternal age at the last menstrual period, preconception BMI, education, nationality, occupation, residence, region, parity, hypertension, hyperglycemia, smoking, drinking, periconception folic acid intake, exposure to harmful substances, history of adverse pregnancy outcomes, and serum HBsAg status.

Abbreviation: PTB=preterm birth; MPTB=moderate-to-late preterm birth; VPTB=very preterm birth; EPTB=extremely preterm birth; ALT=alanine aminotransferase; OR=odds ratio; CI=confidence interval; BMI=body mass index.

Figure 3. 

Subgroup analyses of the association between maternal preconception serum ALT concentrations and the risk of PTB among 5,169,812 reproductive-aged women in China from 2013 to 2017.

Note: Covariates in subgroup analyses were consistent with the variables adjusted in the multivariable logistic regression model for all participants, with the exception of the grouping variable.

Abbreviations: PTB=preterm birth; ALT=alanine aminotransferase; BMI=body mass index; HBsAg=hepatitis B surface antigen; OR=odds ratio; CI=confidence interval.