Fertility treatments

There is no persuasive connection between fertility medications and breast cancer.

Phytoestrogens and soy

Phytoestrogens such as found in soybeans have been extensively studied in animal and human in-vitro and epidemiological studies. The literature support the following conclusions: Plant estrogen intake, such as from soy products, in early adolescence may protect against breast cancer later in life.
Plant estrogen intake later in life is not likely to influence breast cancer incidence either positively or negatively.

It seems reasonable to conclude that soybean-based phytoestrogens are not a major contributor to the incidence of breast cancer.