Vigorous exercise on a regular basis also works to delay the onset of menarche. These factors reduce the risk of breast cancer by closing the oestrogen "window" sooner than may have otherwise occurred (see below).

Pregnancy and breastfeeding

In the context of breast cancer risk, only when a woman goes through a full term pregnancy do her breasts reach full development. Until then, the immature breast cells are more susceptible to abnormal changes stimulated by oestrogen, as well as by a wide range of cancer causing pollutants. Studies show that breast tissue is especially sensitive to exposure to environmental carcinogens that occur between a woman's first period and her first pregnancy.

That is one reason why bearing a child helps reduce the risk of breast cancer particularly premenopausal cancer for most women. Childless women over the age of 45 run a risk from 20 to 70 per cent higher than that of women who have given birth (Epidem Reviews, 1993; 15: 36-47). Experimental studies show that mature breast cells are 50 per cent less susceptible to the risk of breast cancer induced by a chemical carcinogen than are immature cells.

Another reason pregnancy reduces the risk of breast cancer is that the period during and after gestation effectively closes the oestrogen window. Although there is an initial oestrogen surge at the very beginning of pregnancy, oestrogen levels decrease quickly and then progesterone levels soar. High levels of progesterone help to mature the breast cells and bring them to a more stable and quiescent state as they prepare for milk production. These mature cells reproduce more slowly and thus are more resistant to carcinogenic influences. Furthermore, during pregnancy and for several months after full term birth, breast fluid contains lower amounts of oestrogen than are usually present.

Because each pregnancy you carry to term further matures your breast tissue, the more children you have, the lower your risk of breast cancer. Women who have at least five full term pregnancies have half the risk of those who never bear children (Soz Praventivemed, 199; 34: 101-7). An important exception to this is for those women with a family history of breast cancer. Early, repeated pregnancies seem to increase the risk for them.

In 1995, the latest in a long litany of studies showed that women who breastfeed for 17 months or longer had a reduction in breast cancer risk of about 30 per cent (Cancer Causes and Control, 1995; 6: 199-208). When your baby suckles at your breast, that action sends a signal to the hypothalamus, which then inhibits the release of oestrogen and other hormones, thus effectively closing the oestrogen window while you breastfeed and for a number of months afterward. Studies also show that breastfeeding reduces the levels of carcinogens that concentrate in the breast and other body fat. A 1994 study of Canadian women who breastfed their babies found that the longer they had done so, the lower the levels of DDE (dichlorodiphenyldi-chloroethylene), a byproduct of the pesticide DDT, in their breast milk (J Nat Cancer Inst, 1994; 86: 803). DDE concentrations in women who'd never breastfed were about twice those of women who had breastfed 12 months or longer. Other studies show that this reduction becomes more pronounced, the more children a woman bears and breastfeeds.

Oestrogen replacement and the Pill