The researchers sent abstractors out to page through dusty volumes of notes handwritten by Swedish midwives, who had recorded problems during pregnancy and labor as well as the weight of newborns. The researchers obtained birth records for 1,068 women who had developed breast cancer. They compared those women to 2,727 women of the same age who did not get this cancer. Statistical analysis revealed a sharply reduced risk of breast cancer among women whose mothers had developed a pregnancy-induced high blood pressure called toxemia. Such women showed nearly a 60 percent decrease in breast cancer risk.
Research suggests that toxemia of pregnancy results in low concentrations of estrogen.
The disorder involves many perturbations in hormone balance, however, and remains poorly understood, Adami points out. An unknown factor could provide the shield against breast cancer. The team did discover that women who had suffered from jaundice as newborns later had an increased incidence of breast cancer. Jaundice occurs when a newborn's immature liver can't keep up with the production of bilirubin, the orange pigment in bile. Earlier research had suggested that extremely premature infants may have been exposed to high estrogen concentrations in the uterus.