Having children later in life or not at all, combined with a trend in obesity may increase risk of a breast cancer that is hard to detect.
A University of Wisconsin School of Medicine and Public Health research team led by Carbone Cancer Center scientists Polly Newcomb and Amy Trentham-Dietz conducted a study of about 50,000 women born between 1912 and 1986.
The study found that a certain type of breast cancer was more closely associated with women who never had children or those who waited until 30 or later to have a baby.
In addition, the scientists found that obesity could make the risk even higher. These are factors that have become increasingly more common over time.
"Our research found obese women who never had children or had children at 30 or later have a three times greater risk of developing lobular breast cancer," Newcomb said. "There already is strong evidence that obesity, a sedentary lifestyle and alcohol use can be risk factors for breast cancer. Those are all risk factors that women can modify."
For women who were 30 or older when they had their first child or never had a child, there was a 2.5 percent increase in lobular breast cancer. The findings were published in Cancer, the American Cancer Society journal.
Lobular breast cancer begins in the lobules where breast milk is produced and is difficult to spot on a mammogram. Ductal breast cancer, a more common breast cancer, starts in the lining of a milk duct and is easier to detect.
While other studies have linked later childbirth to an increased chance of lobular breast cancer, Newcomb's and Trentham-Dietz's research is the largest study in the United States that finds the association and to observe its increased effect with obesity.
Newcomb said one of the key pieces of take-away information for women is that while some breast cancer risk factors may be hard to control, many lifestyle risks can be controlled including weight, exercise and alcohol use.
Many studies, including work by Newcomb and Trentham-Dietz and the Women's Health Initiative research has shown an association between postmenopausal hormone replacement and risk of lobular breast cancer.
"We believe that combined hormone replacement use in the over the past two decades has increased lobular breast cancer risk, and that nulliparity and late age at birth may share similarities with this hormone use."
The researchers said that the decline in use of hormone replacement therapy coincides with a recent decrease in U.S. lobular breast cancer rates.
The 50,000 study participants from Wisconsin, Massachusetts and New Hampshire included 29,000 women who had been diagnosed with breast cancer.
The researchers point out that while ductal breast cancer remains the most common kind, lobular and mixed ductal-lobular breast cancer increased rapidly in the 1990s and now accounts for 20 percent of all cases.