Cancer risks in twins and singletons from twin and non-twin families.

The unique intrauterine environment has been proposed to put twins at increased risk of certain cancers compared to singletons, still large population comparisons have generally indicated lower risks in twins. To improve the understanding of potential twin influence on cancer we compared twins to their singletons siblings, to target a unique twinning influence. Singletons from twin families were contrasted to singletons from non-twin families to further capture potential twin family influence on risk of cancer. Family relations were identified using the Swedish Multi-Generation Register. Among individuals born between 1932 and 1958, 49,156 twins and N = 35,227 singletons were identified from 18,098 unique twin families. All incident cases of specific cancer types were identified in the National Cancer Register up to the end of 2007. Standardized survival functions were estimated using weighted Cox proportional hazard regression and the corresponding cumulative risks plotted against age. Overall, primary cancers were identified in 9% and 18% of all male and female twins, compared to 11% and 19% of their male and female singleton siblings. When specific cancer sites were compared using standardized cumulative risk plots, no consistent statistically significant differences were noted either between twins and singletons of twin families or between singletons of twin and non-twin families. Despite a different intrauterine experience, twinning does not seem to have any greater negative influence on life-time risks of cancer. The findings also indicate that twin family membership has no substantial influence on cancer risks.