Numerous studies have reported that high birth weight is associated with increased risk of breast cancer in adulthood. This raises the question as to whether restricting calorie intake (while maintaining good nutrition) during pregnancy makes sense as a strategy to reduce breast cancer risk in our daughters. However, restricting calories in an attempt to limit birth weight does not appear to be an effective risk reduction strategy.

High birth weight increases risk of breast cancer in adulthood

Women who weighed more than 8.8 lbs at birth have been found to have a significantly higher risk of breast cancer than women who weighed less than 5.5 lbs. The association between birth weight and breast cancer appears to be stronger in premenopausal women. High birth weight is associated with tall stature in childhood and adulthood, which in turn are associated with increased risk of breast cancer. One study found that high birth weight, tall stature as of age 14, low body mass index (BMI) at age 14, and peak growth at an early age all were independent risk factors for breast cancer. Generally speaking, tall, thin girls have a higher risk of breast cancer in adulthood than short, heavy girls.

How does high birth weight increase breast cancer risk?

The underlying biological mechanism that ties high birth weight to breast cancer risk is not well understood it appears to be tied to various hormones. Birth characteristics of the daughter, including birth weight and birth length, are associated with the mother's hormone levels during pregnancy. One study concluded that birth weight might be a proxy for in utero hormone exposure, and could potentially influence breast cancer risk through the long-term effects of pubertal sex hormones on breast tissue development. Other studies have focused on the role of growth hormones such as insulin-like growth factor (IGF). Umbilical cord blood IGF-1 levels were found to be significantly higher among babies in Boston compared to babies in Shanghai in one study, corresponding to the higher risk of breast cancer in Caucasian compared to Chinese women.

Does severe calorie restriction during the prenatal period reduce breast cancer risk?

Given that high birth weight is tied to breast cancer risk, what would be the result if mothers ate very little during their pregnancies? In fact, this scenario has been studied. A famine took place in the Netherlands at the end of World War II from November 1944 until liberation in May 1945. People were surviving on an estimated 400 to 800 calories per day and the diet lacked variety. The study listed at the beginning of this article describes an analysis of the breast cancer rates of the daughters of women who were pregnant during the famine. Daughters of women who endured famine conditions during the first trimester of their pregnancies were found to have 8.3 times the risk of dying of breast cancer during adulthood than daughters not exposed to famine in the womb. The risk of dying of cancer of any type was 2.3 times higher for the famine-exposed women, suggesting that famine during gestation had a particularly powerful influence on breast cancer risk compared to other cancers. The study highlights the need for adequate calorie intake and nutrition during pregnancy.

Effect of mother's BMI before pregnancy

Are overweight women more likely to have daughters who eventually develop breast cancer? There is no clear answer, but the effect of high BMI, if any, appears to be small. One study found that sex hormone binding globulin (which is associated with reduced breast cancer risk) was inversely related to pre-pregnancy BMI and weight gain during pregnancy, and positively related to vegetable and legume intake. The authors concluded that avoiding excess calorie intake and emphasizing plant foods might contribute to reducing the risk of breast cancer in daughters. However, another study that included 814 mothers of nurses with breast cancer reported that the mothers' pre-pregnancy BMI was not associated with the daughters' breast cancer risk during adulthood. Weight gain during pregnancy also was not found to be associated with daughters' breast cancer risk. The authors of this study concluded that the known association between birth weight and breast cancer risk is likely due to factors independent of mothers' pre-pregnancy BMI or weight gain during pregnancy.

Please see our article on how we can protect our daughters from breast cancer during the prenatal period and infancy for more information on diet during pregnancy and others factors that influence risk.