Yes, breast feeding is one of the natural contraceptive methods. It reduces fecundity by affecting the production of certain reproductive hormones. It is known to suppress the production of Gonadotropin Releasing Hormone (GnRH) and Follicle Stimulating Hormone (FSH).
The release of these hormones triggers ovulation. Breast feeding also leads to increased level of prolactin, a hormone that inhibits ovulation.
So, even when a woman ovulates, her likelihood of conceiving is low if she is breast feeding.