Pregnancy is the most significant modifiable factor known for breast cancer risk in women.
Although an initial increase in risk occurs immediately after parturition in women older than 25 years, the overall lifetime risk of breast cancer decreases after pregnancy.
The beneficial effect of pregnancy reducing the lifetime risk of breast cancer is especially if there has been a pregnancy at a young age. The protective effect is reported to be more than 50% if a full-term pregnancy has occurred before the age of 20 years.
But obviously there are many other considerations when deciding when to conceive.
This protection is due to a genetic modification.
There is an exciting very recent research article about this. Fabienne Meier-Abt, Emanuela Milani, Tim Roloff, Heike Brinkhaus, Stephan Duss, Dominique S Meyer, Ina Klebba, Piotr J Balwierz, Erik van Nimwegen and Mohamed Bentires-Al: Parity induces differentiation and reduces Wnt/Notch signaling ratio and proliferation potential of basal stem/progenitor cells isolated from mouse mammary epithelium. Breast Cancer Research 2013, 15:R36 (https://breast-cancer-research.biomedcentral.com/articles/10.1186/bcr3419)
This research result may be able to help develop chemo-preventive strategies that can provide similar protection for who have never had children.