Women who have children earlier in life tend to reap protective benefits from the hormones the body produces, which slightly decreases risk to developing breast cancer, Rinehart said.
If a woman, at the age 20 or younger, has a full-term pregnancy, the likelihood of her developing breast cancer is halved compared to that of a woman whose first pregnancy occurs after 30.
"But we don't encourage young pregnancies," Rinehart said.
Conversely, a woman who has a late pregnancy, or no children at all, will see a modest increase in the chances of developing breast cancer.
Women who have their first child after 35 are 57 percent more likely to develop the disease by age 70, according to a Harvard Medical School study.
During pregnancy, breast cells grow rapidly and rapid hormonal changes occur. While the cells grow, genetic damage in the breast tissue can lead to breast cancer.
Anything that causes abrupt changes in the body's hormones can cause a woman to develop breast cancer, said Jen Burdette, spokeswoman with the American Cancer Society.
"They're all tied into the hormone system," she said.
Increases in estrogen and progesterone have been linked to breast cancer, according to Monica Clark-McGrew, a nurse with Carroll Hospital Center's Center for Breast Health.
"Though rarely negative levels of those hormones can also be a factor," she said.