Abstract
BackgroundFamily size and birth order are known to influence the risk of some cancers. However, it is still unknown whether these effects change from early to later adulthood. We used the data of the Swedish Family-Cancer Database to further analyze these effects.MethodsWe selected over 5.7 million offspring with identified parents but no parental cancer. We estimated the effect of birth order and family size by Poisson regression adjusted for age, sex, period, region and socioeconomic status. We divided the age at diagnosis in two groups, below and over 50 years, to identify the effect of family size and birth order for different age periods.ResultsNegative associations for increasing birth order were found for endometrial, testicular, skin, thyroid and connective tissue cancers and melanoma. In contrast, we observed positive association between birth order and lung, male and female genital cancers. Family size was associated with decreasing risk for endometrial and testicular cancers, melanoma and squamous cell carcinoma; risk was increased for leukemia and nervous system cancer. The effect of birth order decreased for lung and endometrial cancer from age at diagnosis below to over 50 years. Combined effects for birth order and family size were marginally significant for thyroid gland tumors. Especially, the relative risk for follicular thyroid gland tumors was significantly decreased for increasing birth order.ConclusionOur findings suggest that the effect of birth order decreases from early to late adulthood for lung and endometrial cancer.
Highlights
Size and birth order are known to influence the risk of some cancers
Our results agree with already published findings on the influence of birth order and family size in melanoma
As these can be explained by the socioeconomic status this could be an explanation for squamous cell carcinoma which shows an association of risk with birth order and family size
Summary
Size and birth order are known to influence the risk of some cancers It is still unknown whether these effects change from early to later adulthood. As we used the newest update of the Swedish Family Cancer-Database we had more than 75,000 additional registered cancer cases as previous studies with 178,365 cases [19] This updated version included a total of 254,697 of cancer cases in offspring. Differences in risk estimates for individuals diagnosed before and after 50 years of age are useful for identifying the effects of birth order and family size during life. Both influence the childhood environmental and lifestyle. We focused on that aspect, as this has not been analyzed in previous studies
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