We know from studies of leukemia and some other cancers that people who were exposed to certain factors (such as to radiation) while in the womb (in utero) can have a greater risk of cancer as children and adults. More recently, interest has shifted from large one-time exposures to carcinogens (such as to atomic bomb radiation) to other more subtle factors, such as: older maternal age, maternal obesity, maternal pre-eclampsia during childbirth, higher child birthweight, and certain infections. Each of these factors is thought to alter a mother’s estrogen levels, which may in turn prime cells in the baby’s body for future cancer risk. On top of its possible effect on estrogen levels, a high birthweight may also be associated with in utero exposure to high levels of growth hormones, which like estrogen, can increase later risk of cancer.
A recent review of the research on this topic found that higher birthweight was associated with a fairly small increase in the risk of breast cancer, especially in premenopausal women. Specifically, the paper found that a 2.2 pound (1 kilogram) increase in birthweight raised the risk of breast cancer later in life by about 15 percent (study link) (1). This means that women who weighed, for example, 10 pounds at birth, had a 15 percent higher risk of breast cancer compared to women who weighed 7 pounds, 13 ounces.
1. Xue, F. and K.B. Michels, Intrauterine factors and risk of breast cancer: a systematic review and meta-analysis of current evidence. Lancet Oncol, 2007. 8(12): p. 1088-100.