Ann:  I don't know of any study about breastfeeding and cervical cancer,
but there is one about reduced risk of endometrial cancer:  Rosenblatt KA,
Thomas DB.  Prolonged lactation and endometrial cancer.  WHO Collaborative
Study of Neoplasia and Steroid Contraceptives. Int J Epidemiol Jun
1995;24(3):499-503.
As for ovarian cancer, there are two:
Gwinn ML, Lee NC, Rhodes Ph, et al.  Pregnancy, breast feeding, and oral
contraceptives and the risk of epithelial ovarian cancer. Journal of
Clinical Epidemiology 1990;43:559-68.
Rosenblatt KA, Thomas DB.  Lactation and the risk of epithelial ovarian
cancer.  The WHO Collaborative Study of Neoplasia and Steroid
Contraceptives.  International Journal of Epidemiology 1993;22:192-7.
Hope this helps, Alicia Dermer, MD, IBCLC.