BACKGROUND: It is known that cesarean birth affects maternal outcomes in subsequent pregnancies, but specific effect estimates are lacking. We sought to quantify the effect of cesarean birth reduction among nulliparous, term, singleton, vertex (NTSV) births (i.e., preventable cesarean births) on severe maternal morbidity (SMM) in the second birth. METHODS: We examined birth certificates linked with maternal hospitalization data (2007-2019) from California for NTSV births with a second birth (N = 779,382). The exposure was cesarean delivery in the first birth and the outcome was SMM in the second birth. We used adjusted Poisson regression models to calculate risk ratios and population attributable fraction for SMM in the second birth and conducted a counterfactual impact analysis to estimate how lowering NTSV cesarean births could reduce SMM in the second birth. RESULTS: The adjusted risk ratio for SMM in the second birth given a prior cesarean birth was 1.7 (95% confidence interval: 1.5, 1.9); 15.5% (95% confidence interval: 15.3%, 15.7%) of this SMM may be attributable to prior cesarean birth. In a counterfactual analysis where 12% of the California population was least likely to get a cesarean birth instead delivered vaginally, we observed 174 fewer SMM events in a population of individuals with a low-risk first birth and subsequent birth. CONCLUSION: In our counterfactual analysis, lowering primary cesarean birth among an NTSV population was associated with fewer downstream SMM events in subsequent births and overall. Additionally, our findings reflect the importance of considering the cumulative accrual of risks across the reproductive life course.