Background: Ultraviolet radiation (UVR) exposure is the main risk factor for cutaneous malignant melanoma (CMM), but its specific effect in infancy is unknown. We examined whether season of birth, a proxy for solar UVR exposure in the first few months of life, is associated with CMM in childhood through young adulthood.
Methods: National cohort study of 3 571 574 persons born in Sweden in 1973–2008, followed up for CMM incidence through 2009 (maximum age 37 years) to examine season of birth and other perinatal factors.
Results: There were 1595 CMM cases in 63.9 million person-years of follow-up. We found a sinusoidal pattern in CMM risk by season of birth (P = 0.006), with peak risk corresponding to birthdates in spring (March–May). Adjusted odds ratios for CMM by season of birth were 1.21 [95% confidence interval (CI), 1.05–1.39; P = 0.008] for spring, 1.07 (95% CI, 0.92–1.24; P = 0.40) for summer and 1.12 (95% CI, 0.96–1.29; P = 0.14) for winter, relative to fall. Spring birth was associated with superficial spreading subtype of CMM (P = 0.02), whereas there was no seasonal association with nodular subtype (P = 0.26). Other CMM risk factors included family history of CMM in a sibling (>6-fold) or parent (>3-fold), female gender, high fetal growth and high paternal education level.
Conclusions: In this large cohort study, persons born in spring had increased risk of CMM in childhood through young adulthood, suggesting that the first few months of life may be a critical period of UVR susceptibility. Sun avoidance in early infancy may play an important role in the prevention of CMM in high-risk populations.