Pregnancy during breast cancer: does a mother's parity status modify an offspring's mortality risk?

To assess whether children born to primiparous women around the time of a breast cancer diagnosis have an increased mortality risk. From the merged Swedish Multi-Generation and Cancer Registers, we identified 49,750 eligible children whose mother was diagnosed with breast cancer between 1958 and 2010. Mortality rates in offspring were compared to the background population using standardized mortality ratios (SMR), adjusted for calendar year of birth, attained age, and sex, and calculated for each category of timing of delivery (before, around, or after mother's diagnosis) and mother's parity status. Hazard ratios were assessed using a Cox proportional hazards model and adjusted for socioeconomic status, year of birth and mother's age at birth. Children born to a primiparous woman around a breast cancer diagnosis had a mortality rate five times greater than the background population (SMR 5.26, 95 % CI 1.93-11.5), whereas children born to a multiparous woman had a twofold increase (SMR 2.40, 95 % CI 1.10-4.55). Children of primiparous women born around diagnosis had an adjusted hazard ratio fourfold to that of children of primiparous women born before their mother's diagnosis (HR 4.29, 95 % CI 1.68-8.91), whereas hazard ratios for children of primiparous or multiparous women born at other times were not statistically significant. Children born to primiparous women around a breast cancer diagnosis have an increased relative mortality risk. Although relative risk is increased, in absolute terms children born from a cancer complicated pregnancy do relatively well. Additional investigations are needed to elucidate the reason(s) underlying this observation before the information can be used to inform patient counseling and clinical care.