Early-Life Exposures and Risk of Multiple Myeloma: A Population-Based Case-Control Study in Australia.

Multiple myeloma (MM) is a hematologic malignancy with few known modifiable risk factors. Early-life exposures may influence immune system development and, in turn, affect cancer susceptibility. We examined whether exposures while in utero or during early childhood were associated with MM risk. We conducted a population- and family-based case-control study with 782 MM cases and 1121 controls. Early-life exposures including maternal smoking, being breastfed, childhood growth patterns, household living arrangements, and pet ownership were self-reported. Multivariable logistic regression models, adjusted for age at enrolment, sex, country of birth, socioeconomic position, rural-urban residence and birth weight, were fitted using multiple imputation by chained equations to handle missing data. Living in a home with five or more children was associated with a lower risk of MM (OR = 0.57, 95% CI: 0.38-0.86). Sharing a bedroom before age 11 years (OR = 0.80, 95% CI: 0.64-1.00) and pet ownership in early childhood (OR = 0.76, 95% CI: 0.60-0.98) were also inversely associated with MM risk. No clear associations were observed for maternal smoking during pregnancy (OR = 1.09, 95% CI: 0.79-1.51), being breastfed (OR = 0.96, 95% CI: 0.72-1.28), or height relative to peers at age 7 or 11 (7 years: OR = 1.10, 95% CI: 0.86-1.41; 11 years: OR = 1.06, 95% CI: 0.83-1.34). Our findings support the hypothesis that early-life immune stimulation through environmental exposures may reduce the risk of MM. Further studies are needed to elucidate the biological mechanisms underlying these associations and to explore potential preventive strategies.