Mayo Clinic Cancer Center Study Finds Female Fraternal Twins at Higher Risk for Breast Cancer

Tuesday, February 01, 2000

Feb. 1, 2000

Mayo Clinic Cancer Center Study Finds Female Fraternal Twins at Higher Risk for Breast Cancer

ROCHESTER, MINN. — A Mayo Clinic Cancer Center scientist found that women who have a female fraternal twin have more than twice the risk of developing postmenopausal breast cancer compared to women who were singletons, after adjustment for major breast cancer risk factors.

The study, to be published in the Wed., Feb. 2, 2000, edition of Journal of the National Cancer Institute, found women who were twins, compared to singletons, were 1.7 times more likely to develop breast cancer. This difference persisted after adjusting for major breast cancer risk factors including age, education, family history of breast cancer, height, obesity, age at first menstrual period, age at first live birth, use of hormone replacement therapy and alcohol use. For female fraternal twins, the relative risk was 2.1. The risk for identical twins was no different from singletons. Although the risk for women with a male twin was elevated, with a relative risk of 1.5 compared to singletons, this risk was not statistically different from that for singletons.

"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. As we begin to better understand this period of life and how it may relate to risk of breast cancer, we may eventually be able to recommend new ways for prevention," says James Cerhan, M.D., a Mayo Clinic Cancer Center epidemiologist and the study's lead author. "However, at this early point in the research, there are no direct clinical or public health applications of this research; women who are twins should not become overly concerned, but should follow, like all women, the recommended guidelines for screening mammography."

"This research was motivated by the observation that maternal estrogen and other hormone levels are substantially elevated in twin pregnancies, particularly dizygotic [fraternal] pregnancies, and that intrauterine exposure to elevated levels of pregnancy hormones may increase adult breast cancer risk," says Dr. Cerhan.

In this study, the investigators from Mayo Clinic, University of Minnesota, Wake Forest University and the University of South Carolina evaluated the association of twin membership and risk of breast cancer in a group of 29,197 postmenopausal Iowa women with no history of cancer. A mailed questionnaire in 1986 determined breast cancer risk factors for these women to establish a baseline, and then a follow-up questionnaire in 1992 determined twin membership, sex of the twin and zygosity (whether they were fraternal or identical twins). In this group of Iowa women, 538 women reported being a twin, of whom 24 percent were identical twins, 63 percent were fraternal twins and 13 percent did not know if they were fraternal or identical twins. The investigators followed the 29,197 women for 11 years through linkage to the Iowa Cancer Registry and found that 1,230 developed breast cancer.

This study confirms limited previous findings that twin membership is associated with increased breast cancer risk, especially for fraternal twins. It is unique in its finding that the risk is highest for female fraternal twins. The study also is one of the largest studies to date to evaluate this question. ### Contact: Lisa Copeland 507-538-0844 (days) 507-284-2511 (evenings) e-mail: newsbureau@mayo.edu

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