A woman who is older when she first menstruates or who is younger when she first gives birth has a lower risk of breast cancer. But scientists were unsure what most affects breast-cancer risk: the early reproductive events themselves or the span of time between them?
In the first study of its kind, Dr. Christopher Li and colleagues found that a longer interval between ages at first menstruation and first pregnancy is associated with higher risk of hormonally sensitive types of breast cancer, particularly among white women.
Analyzing data from 8,082 participants in the Women’s Contraceptive and Reproductive Experiences study, the researchers found women who had 16 years or more between first period and first birth had an increased risk of breast cancer compared to an interval of five years or less.
“Various reproductive events induce changes in the breast that protect it from cancer,” said Li, an epidemiologist in the Public Health Sciences Division. “Before pregnancy occurs, breast tissue is relatively undifferentiated and is more susceptible to carcinogens than is breast tissue that has gone through changes induced by a full-term pregnancy. Since many breast cancers are hormonally related, it is not surprising that we find that the time period when undifferentiated breast tissue is exposed to hormones that the body produces, which begins at menarche and ends at first pregnancy, is associated with breast-cancer risk.”
The findings were published Oct. 26 in the American Journal of Epidemiology. Other Hutchinson Center collaborators include Drs. Kathi Malone, Janet Daling and John Potter. The research was funded the National Institute of Child Health and Human Development and the National Cancer Institute.