Objective: Determine the effects of cancer treatment in pregnancy. Design: Observational retrospective cohort study. Setting: 7 Chinese tertiary A hospitals during 2003–2021. Population: Pregnant patients with cancer. Methods: We conducted multiple logistic regression to determine the influence of various factors on preterm birth and small-for-gestational-age infants, log-binomial regression to analyze temporal changes, and chi-square tests to explore the effects of cancer type/treatment. Main outcome measures: Obstetric outcomes; Neonatal outcomes. Results: Of 204 women, 17% terminated pregnancies; 59% received pre-delivery treatment. Rates of pregnancy termination ([RR]: 0.48, 95% confidence interval [CI]: 0.35–0.67) and iatrogenic preterm births (RR: 0.73, 95% CI: 0.54–0.98) reduced, and that of pre-delivery treatment increased, mainly due to increased rates of surgery (RR: 1.87, 95% CI: 1.31–2.67). Maternal systemic diseases were related to a higher risk of small-for-gestational-age infants ([OR]: 12.02, 95% CI: 1.82–79.43). Chemotherapy with taxanes plus platinum-based agents was related to adverse obstetric outcomes (OR: 1.87, 95% CI: 1.42–2.46, P < 0.05). Thyroid (OR: 0.36, 95% CI: 0.22–0.57) and ovarian cancer (OR: 0.70, 95% CI: 0.50–0.98) were associated with fewer cesarean sections. Thyroid cancer was associated with fetal growth restriction (OR: 5.21, 95% CI: 1.21–22.55). Conclusions: Rates of pregnancy termination in cancer have declined. Taxane plus platinum-based chemotherapy was associated with adverse obstetric outcomes. Cancer type influenced outcomes. Funding: Special Project for clinical Research in health industry of Shanghai Municipal Health Commission; Shanghai Science and Technology Commission. Tweetable abstract: Cancer in pregnancy on obstetric and neonatal outcomes: higher risks of adverse obstetric and neonatal outcomes.