Background Congenital anomalies are common, but the possibility that maternal cancer increases the chance of having a child with a birth defect is not fully understood. Objectives To examine the association between maternal cancer before or during pregnancy and the risk of birth defects in offspring. Methods We conducted a retrospective cohort study of live births in Quebec, Canada, between 1989 and 2022 using hospital data. The main exposure measure was maternal cancer before or during pregnancy. The outcome included birth defects detected in offspring during gestation or at birth. We estimated risk ratios (RR) and 95% confidence intervals (CI) for the association of maternal cancer with birth defects using log‐binomial regression models adjusted for potential confounders. Results In this study of 2,568,120 newborns, birth defects were present in 6.0% and 6.7% of infants whose mothers had cancer before or during pregnancy, respectively, compared with 5.7% of infants whose mothers never had cancer. Cancer during pregnancy was associated with heart (RR 1.58, 95% CI 1.03, 2.44), nervous system (RR 4.05, 95% CI 2.20, 7.46) and urinary defects (RR 1.72, 95% CI 1.01, 2.95). Among specific types of malignancies during pregnancy, breast cancer was the most prominent risk factor for birth defects (RR 1.55, 95% CI 1.02, 2.37). Cancer before pregnancy was not associated with any type of birth defect or with defects overall (RR 1.01, 95% CI 0.92, 1.11). Moreover, no specific type of cancer before pregnancy was associated with an increased risk of birth defects. Conclusions Maternal cancer during pregnancy is associated with the risk of congenital anomalies in offspring, however, cancer before pregnancy is not associated with this outcome.