Researchers have found immune cells which protect the body from disease could also increase the risk of breast cancer.
Laboratory studies on mice, at the University of Adelaide, have found immune cells in the breasts, known as macrophages, change in function during the menstrual cycle.
At that time the immune system is down, allowing cancerous cells to escape detection and survive.
Associate Professor Wendy Ingman said the team hoped the findings would lead to more-targeted research.
"Now that we've identified this window of risk, what we really want to do is figure out how we can close it and if we can figure out that then we can actually prevent women from getting breast cancer," she said.
The next phase of research will focus on immune cells in women.