March 13 (UPI) -- Rising breast cancer rates in the United States during the last 40 years have long been linked with women opting to have children later in life, if at all. An analysis published Friday in JAMA Network Open, however, challenges that idea.
Using the Cancer Tumor Registry, one of the oldest databases of cancer cases in the United States, researchers found that the incidence of breast cancer, at least in Connecticut, has been on the rise since the 1930s.
The analysis revealed that the number of women between 25 and 39 years of age diagnosed with the disease in the state has increased by an average of 0.65 percent per year from 1935 through 2015 -- even as birth rates declined over the same period, particularly from the 1970s onward.
"Many have assumed that the increase in early onset breast cancer has mainly been driven by changes in reproductive patterns," co-author Mary Beth Terry, a professor of epidemiology at Columbia University, told UPI. "Our paper shows that that may not be the case, that there has to be other things going on."
Research suggests that a woman's first pregnancy has two effects on her breast cancer risk -- increasing it in the short-term, but decreasing it over the long-term. Typically, women who have their first child at 35 or younger are considered to be at reduced risk for the disease.
This idea has been reinforced by changes in reproductive habits in the United States since the early 1970s. In 1972, on average, women had their first pregnancy at 21 years of age.
By 2018, that number was 26 years of age.
In addition, women began having fewer children over the same period. In 1966, the per capita birth rate peaked -- at least since the start of the 20th century -- at 2.21 live births per woman. In 2010, that figure dropped to 1.41.
"Our analysis shows that breast cancer rates were going up even before the 'baby boom,'" Terry said. "So, the trends in breast cancer aren't just driven by pregnancy."
However, Terry emphasized that she doesn't believe women should necessarily make any changes to their plans based on her findings. This is a "public health" study, she noted, and more about setting the stage for future research into breast cancer risk than about assessing the risk itself.
"Even though age of first pregnancy is an important risk factor for breast cancer, it's not the only one," Terry said, adding that women may want to consider getting screened -- via mammography -- at an earlier age.