Having children with multiple partners is associated with reduced risk of malignant melanoma: an observation seeking a plausible explanation

OBJECTIVE: We examined the association between the number of partners that mothers and fathers have children with and occurrence of cutaneous malignant melanoma (CMM). METHODS: We conducted a complete registry-based follow-up of all Danish mothers born after 1935 from the birth of their second child until CMM, death, emigration, or end of study in 2002. We conducted a similar follow-up of the corresponding fathers. Incidence rate ratios (IRR) and confidence intervals (CI) were estimated by Poisson regression. RESULTS: This study corroborates that women having children with three or more men are half as likely to have CMM as women who have children with one man: incidence rate ratio (IRR) = 0.51, 95% CI: 0.29, 0.91; having children by two fathers reduces risk among women by 20%: IRR = 0.80, 95% CI: 0.70, 0.91. Fathers with multiple partners tend to face a similar risk reduction. CONCLUSION: The similar patterns of mothers and fathers challenge us to consider and propose likely mechanisms common to both sexes. The patterns of reduced risk have now been reported in two large independent complete population-based studies in Sweden and Denmark.