MINNEAPOLIS (AP) — Fraternal twin sisters face more than twice the risk of developing breast cancer after menopause than women who aren’t twins, a Mayo Clinic researcher reported.
A study to be published Wednesday in the Journal of the National Cancer Institute found that women who were fraternal twins were 2.1 times more likely to develop breast cancer than women who were not. The risk for identical twins was no different from the risk for women born solo.
For women with a male twin, the risk was elevated but not statistically significant, researchers said.
The Centers for Disease Control and Prevention estimates that about 175,000 women are diagnosed with breast cancer yearly and more than 43,000 die from it.
Among the 28,659 postmenopausal women in the 10-year study who were single births, 1,195 or 4.2 percent got breast cancer. Among the 149 who had a fraternal twin sister, 13 or 8.7 percent got breast cancer.
Fraternal twins are produced when the mother releases more than one egg and both are fertilized. Identical twins, which are less common, come formed when one fertilized egg splits early in an embryo’s development, giving both twins an identical genetic makeup.
Women who are twins should not become overly concerned, but should follow the recommended guidelines for breast cancer screenings including regular mammograms, said the lead author, Dr. James Cerhan, an epidemiologist with the Mayo Clinic Cancer Center.
“The importance of this study is that it adds to a small but growing body of research that suggests that intrauterine exposures may influence later breast cancer risk,” Cerhan said.
Cerhan said the study was motivated by previous studies that found that a mother’s estrogen and other hormone levels are substantially higher when she is pregnant with twins, and that intrauterine exposure to elevated levels of these hormones may increase the risk of developing breast cancer as an adult.
The researchers said that to their knowledge, their finding that the relative risk is highest for female fraternal twins has not been reported previously, though earlier studies have associated being a twin with an elevated breast cancer risk.
“This is very interesting,” said Chung-cheng Hsieh, a professor of medicine at the University of Massachusetts Medical Center in Worcester and adjunct associate professor of epidemiology at the Harvard School of Public Health.
Hsieh led a study published in 1992 that also found a higher risk of breast cancer in women who were twins, but noted that he and his colleagues were unable to determine then if their subjects were fraternal or identical twins.
For the new study, researchers with the Mayo Clinic, University of Minnesota, Wake Forest University and the University of South Carolina evaluated postmenopausal Iowa women with no history of breast cancer.
Given the size of the data set, Hsieh said, the researchers appeared to have “very good information” to support their conclusions that fraternal twin sisters face an elevated cancer risk.