Objective <p>We primarily evaluated the association of prehypertension (e.g., systolic blood pressure 130–139&#xa0;mmHg and/or diastolic blood pressure 80–89&#xa0;mmHg), aspirin use, and hypertensive disorders of pregnancy (HDP) with preterm birth and small for gestational age (SGA) in women with gestational diabetes mellitus (GDM), and secondarily explored the moderating role of GDM type in these associations.</p> Methods <p>A retrospective cohort study was conducted between 2021 and 2023. Logistic regression was employed to assess the association of prehypertension, aspirin use, HDP with preterm birth (e.g.,  &lt; 37&#xa0;weeks) and SGA (e.g., birth weight &lt; 10th percentile for gestational age). These associations were further evaluated across GDM types (diet-controlled [A1] versus pharmacologically treated [A2]) to examine potential heterogeneity. Path analysis was conducted to decompose the total effect of prehypertension into its direct effect on the outcomes and its indirect effects mediated through aspirin use or HDP, with corresponding effect size estimated.</p> Results <p>A total of 3619 eligible GDM women were enrolled. Compared with women with normal blood pressure, those with prehypertension was associated with an increased risk of preterm birth (10.1% vs. 6.3%,&#xa0;adjusted odds ratio [OR], 1.63; 95% confidence interval [CI], 1.25–2.12) but not with SGA. However, the association between prehypertension and SGA exhibited heterogeneity across GDM types (adjusted ORs were 0.69 (95% CI: 0.50–0.94) in A1 and 1.84 (95% CI: 1.15–2.94) in A2, respectively). Path analysis indicated that the association of prehypertension with SGA and preterm birth were mediated through distinct patterns depending on GDM subtypes.</p> Conclusion <p>Prehypertension was associated with a higher risk of preterm birth, while its association with SGA depended on the type of GDM.</p>