According to pro-lifers they do! http://news.yahoo.com/news?tmpl=story&u=/ap/20041109/ap_on_he_me/abortion_cancer

The most infurating part of it is the "science" they use to back it up:

"Joel Brind, a biochemist at Baruch College in New York who advises the Coalition on Abortion/Breast Cancer, noted that a woman's chances of getting breast cancer go down if she gives birth at a relatively young age. He reasons that those who opt for abortion are giving up a chance of reducing their breast cancer risk.

Therefore, he says, abortion increases the risk of cancer."

Yes, it's absolutely true that giving birth young lowers the risk of breast cancer, though if you don't breastfeed the benefit isn't as high. But just because giving birth young decreases the risk, doesn't mean the reverse is true. And the abortion causes the same level of risk of breast cancer as never getting pregnant in the first place.