Young Mothers and Breast Cancer Risk
Previous studies have shown that women who have their first child before age 30 seem to have a lower risk of breast cancer. However, a new study by Danish researchers has gone one step further.
Jan Wohlfahrt and Mads Melbye of the Statens Serum Institut in Copenhagen, writing in the journal Epidemiology, reported that a woman's breast cancer risk continues to decline with each additional child that she has before age 30-but only if she starts before age 25.
The researchers examined national health data on about 1.5 million Danish women, more than 13,000 of whom eventually developed breast cancer. They found that women who had more children were at lower risk for breast cancer than other women, but only if they had their children at a relatively young age.
Specifically, they found that for every 5-year increase in age at a woman's first, second, third and fourth birth, her breast cancer risk climbed roughly 8 percent compared with women who had comparable children at a younger age.
They concluded that a woman's early reproductive years may be a "critical time window" in which any births cut her long-term risk of breast cancer. Conversely, they wrote, multiple childbirths later in life "evidently induce no reduction in risk."
In another study, published in the American Journal of Epidemiology, researchers from the Yale University School of Medicine found that women who breastfeed for an extended period of time appear to dramatically lower their risk of breast cancer. Dr. Tongzhang Zheng and colleagues interviewed more than 700 women in China and found that those who breastfed for at least 2 years cut their risk of developing breast cancer by more than half (54 percent), compared with women who breastfed for less than six months.
Furthermore, they found that women who breastfed for at least 73 cumulative months with different children over the course of their lives also had a much lower risk.
"These data suggest that prolonged lactation reduces breast cancer risk," they wrote. "The longer duration of lactation-whether it is based on breastfeeding of a first child or breastfeeding over a lifetime-leads to a significantly reduced risk of breast cancer."
They suggested that certain reproductive-cycle hormones that have been linked to breast cancer are suppressed during breastfeeding, while other protective compounds may be released during that time.
Their study also reinforced previous studies that found having a first pregnancy at a younger age and starting menstruation at a later age both contribute to a lower breast cancer risk.
Zheng noted that few women in Western industrialized countries breastfeed for more than four months, which may be one of the factors affecting the difference in breast cancer rates between Western and Asian societies. It may also explain why some studies conducted in the U.S. provide conflicting results about the link between breastfeeding and breast cancer risk.
SOURCES:
Epidemiology, January 2001; 12:68-73
American Journal of Epidemiology, January 15, 2001; 152:1129-1135
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