Objectives: To evaluate risks of preterm birth and severe maternal morbidity (SMM) in female adolescent and young adult cancer survivors; assess maternal comorbidity as a potential mechanism; determine whether associations differ by use of assisted reproductive technology (ART). Design: Retrospective cohort Setting: Privately insured females in the U.S. Sample: Female with live births from 2000 to 2019 within OptumLabs®, a U.S. administrative health claims dataset Methods: Log-binomial regression models estimated relative risks of preterm birth and SMM by cancer status and tested for effect modification. Causal mediation analysis based on a counterfactual approach evaluated the proportions explained by maternal comorbidity. Main Outcome Measures: SMM, preterm birth Results: Among 46,064 cancer survivors, 2,440 singleton births, 214 multiple births, and 2,590 linked newborns occurred after cancer. In singleton births, preterm birth incidence was 14.8% in cancer survivors versus 12.4% in females without cancer (aRR 1.19, 95%CI 1.06-1.34); SMM incidence was 3.9% in cancer survivors versus 2.4% in females without cancer (aRR 1.44, 95%CI 1.13-1.83). Cancer survivors had more maternal comorbidities before and during pregnancy; 26% of the association between cancer and preterm birth and 30% of the association between cancer and SMM was mediated by maternal comorbidities. Associations between cancer and outcomes did not differ between ART and non-ART births. Conclusion: Preterm birth and SMM risks were modestly increased after cancer. Significant proportions of elevated risks may be due to increased comorbidities. Prevention and treatment of comorbidities provides an opportunity to improve perinatal outcomes among cancer survivors.