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Scientists have long puzzled over why early pregnancy protects women from breast cancer.  Researchers say early pregnancy protects humans and rodents against breast cancer.  Previous research found that having a child before the age of 20 decreases a woman's risk of breast cancer by half. 

Now a new study using microarray analysis on the breast tissue of mice that have given birth and virgin mice reveals that the decreased Wnt/Notch signaling ratio after pregnancy may be responsible for the reduced breast cancer risk.

Researchers found that genes involved in the immune system and differentiation were up-regulated after pregnancy while the activity of genes coding for growth factors was reduced.

The study also found that the activity of one particular gene Wnt4 was don-regulated after pregnancy.  Researchers say the protein from the Wnt4 gene is a feminizing protein, and that the absence of this protein propels a fetus towards developing as a boy.

Researchers explain that Wnt and Notch are opposing components of a system, which controls cellular fate within an organism. After further analysis, researchers found that genes regulated by Notch were up-regulated. In other words, Notch-stimulating proteins were up-regulated and Notch-inhibiting proteins were down-regulated.

Researchers explain that Wnt/Notch signaling ratio was permanently altered in the basal stem/progenitor cells of mammary tissue of mice by pregnancy.

"The down-regulation of Wnt is the opposite of that seen in many cancers, and this tightened control of Wnt/Notch after pregnancy may be preventing the runaway growth present in cancer," lead researcher Mohamed Bentires-Alj from the Friedrich Miescher Institute for Biomedical Research in Basel, Switzerland said in a news release.

The findings are published in BioMed Central's open access journal Breast Cancer Research.