Older mothers are more likely to start ovulating before that first warning period that most of us get. So, depending on how old you are....
You have to follow the rules exactly. You'll find them in the Dr. Sears Baby Book, and probably in their breastfeeding book, too. Check online; you might find it at askdrsears.com.
In brief, you have to nurse on demand, at least every 4 hours, I believe, and nurse overnight, as well. Even older mothers will probably have the first warning period (the non-ovulation-prompted kind), but there is a faint possibility that, wham, you'll just find yourself pregnant one day.
Your chances of that go up after the 6-month mark, but not by much if you follow the rules. Exactly. Basically, just nurse lots and lots. And read some Dr. Sears.
L.