BACKGROUND: Birth weight has inconsistent associations with colorectal cancer, possibly due to different anatomic features of the colon versus the rectum. The aim of this study was to investigate the association between birth weight and colon and rectal cancers separately.

METHODS: 193,306 children, born from 1936 to 1972, from the Copenhagen School Health Record Register were followed prospectively in Danish health registers. Colon and rectal cancer cases were defined using the International Classification of Disease version 10 (colon: C18.0-18.9, rectal: 19.9 and 20.9). Only cancers classified as adenocarcinomas were included in the analyses. Cox regressions were used to estimate hazard ratios (HR) and 95% confidence intervals (CI). Analyses were stratified by birth cohort and sex.

RESULTS: During 3.8 million person-years of follow-up, 1465 colon and 961 rectal adenocarcinomas were identified. No significant sex differences were observed; therefore combined results are presented. Birth weight was positively associated with colon cancers with a HR of 1.14 (95% CI, 1.04-1.26) per kilogram of birth weight. For rectal cancer a significant association was not observed for birth weights below 3.5kg. Above 3.5kg an inverse association was observed (at 4.5kg, HR=0.77 [95% CI, 0.61-0.96]). Further, the associations between birth weight and colon and rectal cancer differed significantly from each other (p=0.006).

CONCLUSIONS: Birth weight is positively associated with the risk of adult colon cancer, whereas the results for rectal cancer were inverse only above values of 3.5kg. The results underline the importance of investigating colon and rectal cancer as two different entities.