I'm sorry to say the medical literature I find don't support this statement. (And linking to a poster's bare statement is not the same as citing the specific research or article supporting the claim.)
"Pre Milk" is also called colostrum, which starts being produced during the second trimester of pregnancy (week 13 to week 28). Searching PubMed for articles on cancer and colostrum, I don't find any in the first 3 pages saying that colostrum contributes to cancer. I find some suggesting the opposite: Curr Protein Pept Sci. 2009 Aug;10(4):308-38. Molecular and biotechnological advances in milk proteins in relation to human health.
Limiting the search to colostrum and breast cancer, I find 13 articles, none of which conclude that colostrum contributes to breast cancer.
If you have scientific support for this statement, I'd like to see it.