Risk of obstetric and perinatal complications in women presenting with breast cancer during pregnancy and the first year postpartum in Sweden 1973-2017: A population-based matched study.

Introduction: For women presenting with breast cancer during pregnancy, treatment guidelines were historically restricted to only surgical treatment. Over the past decades, chemotherapy administered during pregnancy has been gradually introduced. Although breast cancer treatments during ongoing pregnancy have been deemed safe, detailed information on potential obstetric risks is lacking. We aimed to assess the risk of adverse obstetric and perinatal outcomes of breast cancer in pregnancy and within 1 year postpartum and in relation to trimester at breast cancer diagnosis, tumor stage, and cancer treatment during pregnancy.

Material And Methods: Population-based matched study. Women diagnosed with breast cancer during pregnancy in 1973-2017 were identified in the Swedish Cancer Register and the Medical Birth Register, with additional information from the National Quality Register for Breast Cancer. Each birth with maternal breast cancer (n = 208 pregnant, n = 672 postpartum) was matched by age, calendar year, and birth order to 10 unexposed births from cancer-free women in the population (n = 2080 and n = 6720). Adjusted conditional logistic and multinomial regression models were used to estimate odds ratios and relative risk ratios, commonly denoted relative risks (RR) with 95% confidence intervals (CI), of adverse obstetric and perinatal outcomes.

Results: Breast cancer during pregnancy was associated with higher risks of preterm birth, both planned (RR 67.1, 95% CI 33.2-135.6) and spontaneous preterm birth (RR 3.8, 95% CI 2.0-7.5), and low birthweight (<2500 g: RR 7.5, 95% CI 4.9-11.3). The associated risks were higher if the breast cancer was diagnosed in the second trimester, and of similar magnitude irrespective of stage and treatment groups. There was a higher risk of low birthweight for gestational age (<25th centile) if breast cancer was diagnosed in the first trimester (RR 2.8, 95% CI 1.1-7.3). Risks of other pregnancy complications were similar to those of unexposed women, as were risks of neonatal mortality and malformations. Postpartum breast cancer was only associated with bleeding during pregnancy (RR 1.6, 95% CI 1.0-2.8).

Conclusions: Preterm birth and related adverse outcomes were more common in women diagnosed with breast cancer during pregnancy. Reassuringly, breast cancer was not associated with other maternal pregnancy complications or adverse outcomes in children.