Parity, age at first birth and risk of death from kidney cancer: a population-based cohort study in Taiwan.

Background: This study was undertaken to examine whether there is an association between parity and age at first birth and risk of kidney cancer.

Methods: The study cohort consisted of 1 292 462 women who had a first and singleton childbirth between 1 January 1978 and 31 December 1987. We tracked each woman from the time of her first childbirth to 31 December 2009, and their vital status was ascertained by linking records with the computerized mortality database. Cox proportional hazard regression models were used to estimate the hazard ratios (HRs) of death from kidney cancer associated with parity and age at first birth.

Results: There were 95 kidney cancer deaths during 34,980,246 person-years of follow-up. The mortality rate of kidney cancer was 0.27 cases per 100,000 person-years. The adjusted HR was 1.88 [95% confidence interval (CI) 1.10-3.19] for women who gave birth between 24 and 26 years of age and 2.52 (95% CI 1.44-4.40) for women who gave birth after 26 years of age, when compared with women who gave birth when <23 years of age. A trend of increasing risk of kidney cancer was seen with increasing age at first birth. The adjusted HR was 0.88 (95% CI 0.49-1.59) for women who had two children and 0.89 (95% CI 0.47-1.67) for women with three or more births, when compared with women who had given birth to only one child.

Conclusions: This study is the first to suggest that early age at first birth may confer a protective effect on the risk of kidney cancer.