Gestational age and cancer risk up to young adulthood in Swedish population born 1974 to 2013: A population-based cohort study.

We examined the association between gestational age and risk of any primary cancer and observed whether the risk patterns differed by sex, birth weight for gestational age categories, cancer site and age of onset. All people live-born in Sweden 1974 to 2013 were prospectively followed up from birth until 2016 using national registers. Gestational age was extracted from the Medical Birth Register and primary malignant cancer diagnoses were from the Swedish cancer register. The adjusted hazard ratios (aHR) for any primary cancer according to weekly gestational age and gestational age categories were determined using cox proportional hazards models adjusted for birth year and parental age. The study included 3 137 691 people; 180 363 (5.8%) born preterm and 254 790 (8.1%) born postterm. They were followed up for 71 691 112 person-years, to a maximum of 43 years and recorded 22 604 new cancers. Although aHRs for the predefined GA categories were only increased for moderate to late preterm delivery (aHR 1.07, 95% CI 1.01-1.14), gestational week-specific aHRs were increased for gestational weeks 30 to 35, with greatest aHR observed for 31 weeks (aHR 1.18, 95% CI 1.05-1.32). Increased cancer risk related to shorter gestational ages were observed particularly for women, those born small for gestational age, childhood cancers and for cancers originating at certain sites (eg, testicular and liver cancer). We provide the first evidence that those born between 30 and 35 weeks gestation may have increased risk of any primary malignant cancer up to young adulthood. Additionally, increasing gestational ages may reduce the risk of testicular and liver cancer.