Breast cancer is the most common cancer among women in China, according to the latest data from China’s national cancer registry. An analysis of the data reveals that the cancer has increased at a rate of around 3.5% a year from 2000 to 2013, compared with a drop of 0.4% a year over the same period in the US.
The analysis also reveals that breast cancer rates are higher in urban areas of China than in rural areas. And the higher the population density, the higher the rate. For small cities (population below 500,000), the incidence of breast cancer is 30 per 100,000. For medium-sized cities (population between 500,000 to 1,000,000), it is 40 in 100,000. And for large cities (population above 1m), the incidence rate is 60 per 100,000 women.
With the rapid development of China’s economy, more and more people have moved from rural areas and towns to large cities. As a result, many “megacities” have sprung up. By 2014, China had six megacities with populations above 10m. It is very likely that urbanisation is having a big impact on breast cancer incidence in China.
Here is a list of some of the factors that may be behind the rise in breast cancer incidence in China:
Childbearing: Having more than one child lowers breast-cancer risk. With the one-child policy in place since 1979, most women – especially if they worked in the city – had to strictly follow the policy in order to avoid being fined. Although the one-child policy rule was replaced in 2015 with a two-child policy rule, the possible benefit on breast cancer incidence will probably take 15 to 20 years to show.
Research also shows that the women who have their first child at age 35 or younger tend to have a protective benefit from pregnancy. However, in China, many women have chosen to delay having a child as a result of work pressure and cultural change.
Women are also less likely to breastfeed than previous generations, which may be another contributing factor. Research has shown that both pregnancy and breastfeeding reduce a woman’s risk of developing cancer, because they reduce the lifetime number of menstrual cycles. As a result, women are exposed to less oestrogen. (Oestrogen can stimulate breast cancer cells to grow.) It has also been hypothesised that breast cells need to mature in order to produce milk and mature cells are more resistant to becoming cancer cells.
Researchers in China have found associations between these “reproductive factors” – including oral contraceptives and hormone replacement therapy (HRT) – and the rise in breast cancer incidence in China.