Mæhle et al. BMC Cancer 2010, 10:115 http://www.biomedcentral.com/1471-2407/10/115 Norwegian Cancer Registry to identify women with breast cancer who were residents in Trondheim or Ber- gen at the time of diagnosis. The Central Person Regis- try, which is administered by Statistics Norway, provided the necessary information on place of resi- dence at birth. That registry maintains continuously updated records on each woman’s residential address and the identity of the mother. The information on birth weight and birth length for patients born in Trondheim was collected from the birth archive of EC Dahl’s Foundation that served as the major birth clinic in Trondheim from 1910 to 1970. Birth data from Bergen were derived from archives of Bergen Birth Foundation and Midwife School, and later from the Maternity Hospital in Bergen. The information is stored at the Regional State Archives in Bergen. We further used information from the Central Person Regis- try of Norway to verify individual linkage between women of the study and their birth records. To be included, patients had to be singletons born at term; birth length was restricted to range from 46 to 55 cm, and birth weight could range from 2000 to 5000 grams. In the analysis, we only included patients who were treated with modified radical mastectomy. Among 373 potentially eligible patients, we could include 331 patients in the analysis for whom we had sufficient information from birth. Perinatal information was abstracted from birth records and included birth weight (grams) and birth length (centimetres). As a measure of body mass, independent of length, ponderal index was calculated as birth weight divided by the cubed value of birth length (kg/length(m)3). The correlation coefficient between ponderal index and birth length was 0.00, and between ponderal index and birth weight it was 0.66, indicating that birth length and ponderal index may be considered independent of each other. Breast cancer specimens were examined at the Depart- ment of Pathology, St Olav’s University Hospital in Trondheim, or at the Department of Pathology, Hauke- land University Hospital in Bergen. Information on tumour diameter and lymph node status at diagnosis was abstracted from the pathology reports. In the statistical analysis, birth length, birth weight and ponderal index were categorised into approximate quintiles, and in subsequent analyses, the second, third and fourth quintile were combined into one category because of the range between the second and the fourth quintile was small. Thus, each variable had three cate- gories: the first category was the lowest quintile; the sec- ond category was a combination of the second, third, and fourth quintile, and the third category was the high- est quintile. For birth weight, the first quintile included weights below or equal to 3050 grams, and the fifth quintile included birth weights above 3850 grams. The Page 2 of 4 corresponding threshold values for the lowest and high- est quintile of birth length were 48 cm and 52 cm, and for ponderal index, it was 25.0 kg/m3 and 29.1 kg/m3. All patients were followed up by linkage to data from the Causes of Death Registry at Statistics Norway. Follow-up started at the date of diagnosis, and median follow-up was 108 months. Women were censored at the time of death, or at the end of follow-up. For patients in Trondheim, follow-up ended on December 31, 1999, and for patients in Bergen, follow-up ended on December 31, 2003. Death from breast cancer was the primary endpoint, and no patient was lost to follow-up. The Cox proportional hazards model was used to estimate the relative risk of dying (hazard ratio) from breast cancer related to the categories (see above) of birth weight, birth length and ponderal index. The Cox model is based on the assumption that death rates between categories of patients are constant over time. No serious deviations from the proportionality assump- tion were found, based on plots of the log minus log survival function (not shown). In a multivariable analy- sis, we adjusted for place of birth and year of diagnosis, and in additional analysis, we also took lymph node status and tumour diameter at diagnosis into account. All statistical analyses were done using SPSS, version 11.0 for Windows (SPSS Inc., Chicago, IL, USA). The study was approved by the regional committee for ethics in medical research. Results Patient characteristics are presented in Table 1, and Table 2 shows the associations of birth size indicators (length, weight and ponderal index) with breast cancer specific survival, after adjustment for place of birth and year of diagnosis. The results indicate that women who were longer than 52 cm at birth had 92% higher risk (hazard ratio 1.92, 95% confidence interval, 1.09-3.41) of dying from breast cancer compared to patients who were 48 cm or shorter. A test for linear trend across birth length provided a p-value of 3%. We found no clear associations with survival related to birth weight or ponderal index. Since ponderal index and birth length play indepen- dent roles, both variables were included in the subse- quent multivariable analyses (Table 3). In addition, we included place of birth and year of diagnosis in the ana- lysis (Analysis A, Table 3). The results were quite simi- lar to those shown in Table 2. In subsequent analyses, we also included lymph node status (positive vs. nega- tive) (Analysis B, Table 3), and tumour diameter (Analy- sis C, Table 3) as co-variates. Information on lymph node status was missing in 39 patients, and tumour dia- meter was missing in 86 patients; and in analysis B in Table 3, we therefore included 293 patients, and in