Abstract
This study was undertaken to examine whether there is an association between parity and age at first birth and risk of colon cancer. The study cohort consisted of 1,292,462 women who had a first and singleton childbirth between 1978 and 1987. We tracked each woman from the time of their first childbirth to December 31, 2009, and their vital status was ascertained by linking records with the computerized mortality database. We used the Cox proportional hazards model with time-dependent covariates to estimate the hazard ratios (HR) of death from colon cancer associated with parity and age at first birth. We limited eligible colon cancer deaths to those who were 45 years old or more to exclude possible heredity colon cancer cases, which usually occur at an early age. There were 670 colon cancer deaths during 34,980,246 person-years of follow-up. The colon cancer death rate was 1.96 cases per 100,000 person-years. The adjusted HR was 2.76 (95% CI = 1.60-4.75) for women who gave birth between 20 and 24 years and 7.35 (95% CI = 4.28-12.62) for women who gave birth after 24 years of age when compared with women who gave birth at younger than 20 years. A rising risk of colon cancer was seen with increasing age at first birth. The adjusted HR were 0.81 (95% CI = 0.65-1.02) among women with two live births, 0.93 (95% CI = 0.74-1.18) among women with three live births and 0.72 (95% CI = 0.51-1.00) for women with four or more births when compared with women who had given birth to only one child. The present study provides evidence that reproductive factors (parity and early age at first birth) may confer a protective effect on the risk of colon cancer.