Women who get breast cancer within five years of giving birth are at much higher risk than those who have never had children of seeing the tumours spread, or metastasize, to the liver. This could be due to changes in the liver as it readjusts after pregnancy.
In rodents, the liver expands during pregnancy. Virginia Borges at the University of Colorado, Aurora, Pepper Schedin at Oregon Health and Science University in Portland and their colleagues studied the organ in rats and mice. They found that after weaning, the liver shrank and was infiltrated with immune-suppressing white blood cells. Many liver cells died and others secreted proteins that promote wound-healing, providing a fertile ‘soil’ in which tumours can grow.
When the authors injected cancer cells into mice, more liver metastases developed in animals that had just weaned their pups than in those that had never had offspring.