Objectives
To investigate the obstetrical management of cancer in pregnancy and to determine adverse pregnancy and neonatal outcomes.

Design
A nationwide cohort study.

Setting and population
We included all pregnancies (n = 4 071 848) in Denmark from 1 January 1973 to 31 December 2018.

Methods
Exposure was defined as pregnancies exposed to maternal cancer (n = 1068). The control group comprised pregnancies without cancer. The groups were compared using logistic regression analysis and adjusted for potential confounders.

Main outcome measures
The outcomes were induced abortion, preterm birth and adverse neonatal outcomes.

Results
More women with cancer in pregnancy, as compared with the control group, experienced induced abortion (24.8% vs. 20.0%); first-trimester induced abortion adjusted odds ratio (aOR) 3.5 (95% confidence interval [CI] 2.7–4.5), second-trimester induced abortion; aOR 8.8 (95% CI 6.3–12.3), planned preterm birth (11.8% vs. 1.3%); aOR 10.8 (95% CI 8.0–14.6) and planned preterm birth at <32 gestational weeks; aOR 16.3 (95% CI 8.3–31.7). Neonates born to mothers with cancer in pregnancy had a higher risk of respiratory distress syndrome; aOR 3.5 (95% CI 2.8–4.4), low birthweight; aOR 3.8 (95% CI 3.1–4.8), admission to neonatal intensive care unit for >7 days; aOR 5.1 (95% CI 3.9–6.6), neonatal infection; aOR 1.8 (95% CI1.1–3.1) and neonatal mortality; aOR 4.7 (95% CI 2.7–8.2), but not of SGA; aOR 1.0 (95% CI 0.6–1.5) and malformations; 1.2 (95% CI 0.9–1.7).

Conclusion
Cancer in pregnancy increases the risk of induced abortion and planned premature birth. Neonates born to mothers with cancer in pregnancy had an increased risk of neonatal morbidity and mortality, presumably due to prematurity.