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.

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The drug does cause side-effects in some patients, including, rarely, cancer of the lining of the womb, but the scientists said that the increased chance of preventing breast cancer returning far outweighed the risk.

BBC: Breast cancer