The protective effect of an early pregnancy is thought to be due to the fact that a first, full-term pregnancy stimulates the full differentiation of the cells lining the milk ducts, where most breast cancers arise.

FORBES: After 40 Years Of Research, What Do We Know About Preventing Breast Cancer?

Such periods include: in utero exposure, puberty, and the period between menarche and a first full-term pregnancy.

FORBES: After 40 Years Of Research, What Do We Know About Preventing Breast Cancer?