Birthweight, childhood growth and risk of breast cancer in a British cohort.

We have examined the relationship between birthweight and risk of breast cancer, taking into account growth in childhood, using data on a total of 2221 women born in 1946 and followed up to 1997. Thirty-seven breast cancers occurred during follow-up. There was evidence of greater risk of breast cancer with greater birthweight (rate ratio = 1.76 (95% CI: 0.92, 3.35) for birthweight >/= 3.5 kg vs birthweight < 3.5 kg), which was more marked at pre-menopausal ages (RR = 2.31, 95% CI: 0.93, 5.74). The relation with birthweight was not substantially confounded by any of the measured adult risk factors. A significant interaction was observed between the effects of birthweight and height at age 7 years. Relative to those born lighter than 3.5 kg, women who were heavy at birth (>/= 3.5 kg) and short or average at 7 years (< 1.22 m) had a 21% increase in breast cancer rates (RR = 1.21; 95% CI = 0.49-2.99), while women who were heavy at birth (>/= 3.5 kg) but tall at 7 years (>/= 1.22 m) had a four-fold increase (RR = 4.01; 95% CI = 1.82-8.83). These results suggest that the effect of birthweight on breast cancer risk may be modulated by childhood growth. Copyright 2000 Cancer Research Campaign.