Thursday, July 15, 2010

Later age at first birth and increase in breast cancer risk

A British media story today points to later age at first birth as a major cause of increasing breast cancer rates in the UK (see story). This is not news, though increasing knowledge about causes of breast cancer is useful. We have known for decades that the later a women ahs her first birth the larger the adverse effect of this birth on her lifetime risk of breast caner. In fact, if she has only one birth say at age 35 her risk is higher for life by about 16% compared to a women who never has children, on the other hand, if she has her first birth at age 20 and has 4 children spaced 3 years apart, then her risk is 27% lower than the woman who ahs no children (Colditz and Rosner 2000). Thus age at childbirth and number of children together play an important part in driving the population risk of breast cancer.

Traditional low income countries have an added factor at play – age at menarche or first menstrual period. In low income countries and before industrialization average age of onset of menstrual periods is typically around 17 – and so the time from menarche to first broth is shorter than in our post industrial civilization where menarche is now at 12 or younger, on average, and age at first birth is rising – to higher than 30, on average, in Holland.

Asia as an example of rapid changes in risk factors and breast cancer incidence
We have previously estimated the increase in breast cancer incidence in China expected due to the changes in reproduction among women (Linos, Spanos et al. 2008). Likewise, reports from Singapore (Seow, Duffy et al. 1996) and Hong Kong point to secular trends in breast cancer incidence as women progress through economic transition. Recent media attention has pointed to the rapid decline in fertility in Korea. As the Economist noted, this decline in Korea from 6 births per 1000 women in 1965 to 2 in 1980 is more rapid than seen in Europe- for example Britain decreased this same magnitude over a  span of 120 years.

Also consistent with a long transition of industrialization impacting reproductive profiles, age at menarche has decreased quite rapidly in Korea. The third Korean National Health and Nutrition survey included 3562 women born from 1920 to 1985. Mean age at menarche decreased from 16.90±1.25 years for women born between 1920 and 1925 to 13.79±1.37 years for those born between 1980 and 1985, indicating a downward trend of 0.68 years per decade (95% CI, 0.640.71) in age at menarche. Mean age at menarche of girls born between 1986 and 1995 was 13.10±0.06 years.

Ahn and colleagues note the increase in breast cancer in Korea (Ahn and Yoo 2006). The median age at diagnosis is 47. Over the 8-year period form 1996 to 2004 the proportion of cases with menarche less than 13 rose from 8 to 13%. In a series of 5001 cases treated from 1989 to 2004 (Son, Kwak et al. 2006) the median age at diagnosis rose from 44 in 1991 to 46 in 2003 and the proportion of cases ER positive was 57.1% and PR positive 50.8% . For women in Korea, ages 45-49 the rate of breast cancer in 2002 was 90 cases per 100,000 women. This is still lower than that for white or Asian women in the USA, where the national registry data show a rate of around 60 per 100,000 women ages 45 to 49.


Literature cited

Ahn, S. H. and K. Y. Yoo (2006). "Chronological changes of clinical characteristics in 31,115 new breast cancer patients among Koreans during 1996-2004." Breast Cancer Res Treat 99(2): 209-214.
Colditz, G. and B. Rosner (2000). "Cumulative risk of breast cancer to age 70 years according to risk factor status: data from the Nurses' Health Study." Am J Epidemiol 152(10): 950-964.
Linos, E., D. Spanos, et al. (2008). "Effects of Reproductive and Demographic Changes on Breast Cancer Incidence in China: A Modeling Analysis." J Natl Cancer Inst 100: 1339-1341.
Seow, A., S. Duffy, et al. (1996). "Breast cancer in Singapore: Trends in incidence 1968-1992." Int J Epidemiol 25: 40-45.
Son, B. H., B. S. Kwak, et al. (2006). "Changing patterns in the clinical characteristics of Korean patients with breast cancer during the last 15 years." Arch Surg 141(2): 155-160.


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