Purpose: To examine whether maternal cancer during pregnancy is associated with increased risks of stillbirth and infant mortality.

Methods: On the basis of nationwide health registers, we conducted a study of 3,947,215 singleton births in Sweden from 1973 through 2012. Exposure was defined as maternal cancer diagnosed during pregnancy (number of births = 984) or during the year after pregnancy (number of births = 2,723). We calculated incidence rate ratios (IRRs) for stillbirth and infant mortality, comparing exposed births to unexposed births. Small-for-gestational-age (SGA) and preterm births were examined as secondary outcomes.

Results: Maternal cancer diagnosed during pregnancy was positively associated with stillbirth (IRR, 2.5; 95% CI, 1.2 to 5.0), mainly stillbirths assessed as SGA (IRR, 4.9; 95% CI, 2.2 to 11.0), and with preterm SGA births (relative risk 3.0; 95% CI, 2.1 to 4.4). Positive associations of maternal cancer diagnosed during pregnancy or the year after pregnancy were noted for both neonatal mortality (deaths within 0 to 27 days; IRR, 2.7; 95% CI, 1.3 to 5.6 and IRR, 2.0; 95% CI, 1.2 to 3.2, respectively) and preterm birth (IRR, 5.8; 95% CI, 5.3 to 6.5 and IRR, 1.6; 95% CI, 1.4 to 1.8, respectively). The positive association with preterm birth was due to iatrogenic instead of spontaneous preterm birth. Preterm birth explained 89% of the association of maternal cancer during pregnancy with neonatal mortality.

Conclusion: Maternal cancer during pregnancy is associated with increased risks of rare but fatal outcomes, including stillbirth and neonatal mortality. This may be due to conditions associated with fetal growth restriction and iatrogenic preterm birth. Careful monitoring of fetal growth and cautious decision making on preterm delivery should therefore be reinforced.