Some women worry because one or more pregnancies may have terminated early, either through abortion or because of a miscarriage. They fear that this may put them at a higher risk for breast cancer.

An article from Medical News Today is reassuring:

Women younger than age 35 who carry a pregnancy to term appear to have a reduced lifetime risk of breast cancer, according to background information in the article. Pregnancy may accelerate breast cell differentiation, the process by which cells take on specialized roles. "An incomplete pregnancy may not result in sufficient differentiation to counter the high levels of pregnancy hormones that may foster proliferation," the rapid growth and division typical of cancer cells, the authors write. "However, these biological mechanisms are uncertain, and a prematurely terminated pregnancy may not affect breast cancer risk at all."

This is an old myth, as explained in Scientific American:

The link between abortion and breast cancer is an old canard that gets trotted out often enough that it's worth addressing for the nth time. The National Cancer Institute, part of the NIH, has long declared this link bogus.