Objective To investigate the causal association among glycemic traits, plasma metabolites, and adverse pregnancy outcomes (APOs), as well as potential mediating mechanisms, using Mendelian randomization (MR).

Methods Based on summary data from genome-wide association studies, a two-sample MR framework, including sensitivity analyses, reverse MR, and two-step mediation analysis was applied. Four glycemic traits (fasting glucose, fasting insulin, 2 h postprandial glucose, and HbA1c) and 1,400 plasma metabolites were used as exposures, while four APOs (spontaneous abortion, fetal growth restriction, pre-eclampsia/eclampsia, and preterm birth) served as outcomes. The inverse variance weighted method was used as the primary analysis, supplemented by various sensitivity analyses to verify the robustness of the results.

Results MR analysis showed that genetically predicted higher fasting glucose was causally associated with a reduced risk of fetal growth restriction [OR=0.660, 95%CI (0.496, 0.879), P=0.004]. Higher fasting insulin was a risk factor for pre-eclampsia/eclampsia [OR=1.738, 95%CI (1.057, 2.859), P=0.030], while higher 2 h postprandial glucose was associated with an increased risk of preterm birth [OR=1.172, 95%CI (1.014, 1.354), P=0.032]. No significant causal associations were found between HbA1c and any of the APOs in the MR analysis (P > 0.05). Metabolite analysis revealed that higher levels of hexanoylglutamine and 3-methylcytidine increased the risk of pre-eclampsia/eclampsia, whereas a higher level of N6-acetyllysine reduced the risk of preterm birth. Mediation analysis further suggested that 3-methylcytidine may partially mediate the effect of fasting insulin on pre-eclampsia/eclampsia (mediation proportion: 4.88%). All primary results were validated by sensitivity analyses, and reverse MR analysis found no evidence of reverse causation.

Conclusion This study provides genetic evidence for the potential differential causal effects of glycemic traits on distinct APOs and, reveals the partial mediating role of 3-methylcytidine in the pathway from fasting insulin to pre-eclampsia/eclampsia, offering new insights for precise glycemic management during pregnancy.

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