Mæhle et al. BMC Cancer 2010, 10:115 http://www.biomedcentral.com/1471-2407/10/115 high birth length that we found in our study is likely to be underestimated. In Norway, however, all cancer treat- ment is free of cost, and this is likely to reduce the potential for bias due to differences in access to and quality of treatment. Another possible source of bias could be that the association of birth length with adult socioeconomic sta- tus, including higher education, could lead to earlier diagnosis in women with high birth length. This possibi- lity could result in lead time bias, where survival from diagnosis to death would be longer among patients with high birth length compared to women with shorter birth length, who may be diagnosed at a later stage. Also under this scenario, however, the poorer outcome in patients with high birth length is likely to be underestimated. The secular increase in birth length that took place in the 20th century may also be a concern. In an attempt to take effects of calendar time (period) into account, we adjusted for year of birth, but this did not influence the estimates of effect. However, this adjustment may not be sufficient to account for secular changes in birth length. It is also possible that important but unmeasured con- founding factors could explain the poorer outcome of patients with high birth length. For such a factor to be of practical interest, however, it would have to be strong, and substantially influence breast cancer survival. Few known prognostic factors meet these criteria, and they are typically variables that describe the stage of the disease at the time of diagnosis, and therefore, these are not factors that could influence birth lenth. In the analy- sis, we adjusted for lymph node status and tumour dia- meter, but the adjustment did not substantially influence the results. However, body weight is a factor that does influence the prognosis of breast cancer patients [9]. Birth length is both associated with adult height and weight [11,12], but it is not clear if adult weight should be considered as a potentially confound- ing factor in relation to breast cancer survival. Maybe it is more appropriate to consider the effect of adult body size to be a possible mediator on the causal pathway from birth length to the outcome of breast cancer. Conclusions It is difficult to see any practical implication of the find- ing that high birth length is associated with a poorer prognosis of breast cancer. Nonetheless, by establishing a link between conditions that operate in utero and the development and prognosis of later breast cancer, these results may be helpful in guiding investigators towards prognostic mechanisms. The relatively strong association of birth length, indicating longitudinal growth, may sug- gest that the insulin like growth factor (IGF) system is involved in the underlying mechanisms. Page 4 of 4 Acknowledgements This study was supported by the University of Bergen, and by the Norwegian University of Science and Technology. Author details 1Section of Pathology, the Gade Institute, University of Bergen, Bergen, Norway. 2Department of Pathology, Haukeland University Hospital, Bergen, Norway. 3Department of Public Health, Norwegian University of Science and Technology, Trondheim, Norway. 4The Cancer Registry of Norway, Oslo, Norway. Authors’ contributions BOM and LJV collected the data, and all three authors have contributed to the analysis of the data, interpretation of the findings, and to the writing of the manuscript. All authors have seen and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 3 September 2009 Accepted: 26 March 2010 Published: 26 March 2010 References 1. 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