This month Australian researchers conducted a case control study in China about the relationship between ovarian cancer risk and breastfeeding. The study showed "significant dose response relations" for both "prolonged breastfeeding" and "number of children." According to the data, those who breastfeed for 31 months or more decrease their risk significantly compared to those that breastfeed for 10 months or less. Also, those who breastfeed 3 or more children decrease their risk of ovarian risk significantly compared to those who breastfeed 1 child. Other studies show similar results.
It is the reduction in ovulations that seems to provide the protection against ovarian cancer. Also, the more ovulation is reduced by pregnancy and breastfeeding, the greater the effect. The nice thing about having children and breastfeeding is that you are not introducing unnatural chemicals or hormones that can harm a woman and/or her unborn baby if she would become pregnant. God instilled a natural way to reduce cancer right into a woman's body. I guess having children and practicing ecological breastfeeding are good for women's health after all :)