A recent study of breast cancer has found that:
Abortion increases risk by over 600%.
Birth Control Pills increase risk by 950%.
Less Breastfeeding increases risk by 15%.
Having first child after 21 increases risk by over 200%.
Having last child after 27 increases risk by over 300%.
All these findings are consistent with previous studies.
Abortion Causes Cancer
The history of abortions was found to be significantly higher in breast cancer cases as compared to controls, with the Relative Risk (RR) = 6.26 in women having a history of abortion. Results of other studies are not conclusive, while some reporting an increase in risk with induced abortion,  others reporting a decrease in risk of breast cancer,  and few studies reporting no association.
Birth Control Pills Cause Breast Cancer
In the present study, the risk of breast cancer was 9.50 times higher in women having a history of consumption of oral contraceptive pills. Previous studies have also shown similar results.
Longer Lactation (Breastfeeding) Prevents Breast Cancer
An association between lactation and protection from breast cancer has been postulated for a long time.  The results of the present study also revealed similar association, with breast cancer cases reporting a lower mean duration of breastfeeding (11.16 months) as compared to controls (21.00 months). Studies conducted in different countries have also reported similar findings. ,,, The RR of breast cancer was found to increase 14.9 (95% confidence interval: 8.69, 25.7) times in women having mean duration of breastfeeding less than 13 months. It has been suggested that lactation might reduce breast cancer risk by temporarily draining the breasts of potential chemical carcinogens and finally, the hormone oxytocin, which causes contraction of myoepithelial cells as a response to suction, has been reported to inhibit cell proliferation and tumor growth in animal models. Lactation also has a direct physical effect on the breast, such as changes in breast ductal epithelial cells leading to mechanical “flushing-out” of carcinogens.
Earlier Age of First AND Last Pregnancy Prevents Breast Cancer
It was observed that breast cancer cases married at a later age as compared to the controls and hence had their first child at a later age. Women who had an age of marriage more than 20 years had a 2.69 (95% confidence interval: 1.77, 4.07) times higher risk of breast cancer. Similarly, the risk of breast cancer was twofold higher in women having their first child at more than 21 years of age. Similar results were obtained in a study from India in which the OR was 2.4 (95% confidence interval: 1.3, 4.4) when the age at marriage was more than 20 years. Other studies have also revealed similar findings. ,,,
It has been speculated that a full-term pregnancy at an early age may reduce the likelihood of tumor initiation, while a full-term pregnancy at a later age may promote the growth of existing tumor cells.  Pregnancy induces terminal differentiation of human breast glands, which may have a smaller proliferative component. The first pregnancy induces irreversible changes that either render the breast tissue itself less susceptible to induction of cancer or reduced the carcinogenic stimulus to the breast.
The present study revealed that the breast cancer cases had statistically higher mean age at last childbirth as compared to the controls. The risk of breast cancer increased 3.29 (95% confidence interval: 2.15, 5.02) times in women having age at last childbirth more than 27 years.
Bhadoria A S, Kapil U, Sareen N, Singh P. Reproductive factors and breast cancer: A case-control study in tertiary care hospital of North India. Indian J Cancer [serial online] 2013 [cited 2014 Apr 8];50:316-21. Available from: http://www.indianjcancer.com/text.asp?2013/50/4/316/123606