So, is breast feeding an effective form of birth control? You would think "yes" or "no" would be the simplest answers, but it's not. It's actually a little bit more complicated than that. How often are you breast feeding? Are you giving supplements with formula? Are you having problems with your breast milk production? These are things that are going to have an impact as to whether or not breast feeding is an effective form of birth control for you.
In general, what I tell my patients is if you are exclusively breast feeding, in other words you're not giving any formula or any bottles over the course of the first eight weeks of your postpartum period, then it is an effective form of birth control. After the first eight weeks, there's a big question mark as to whether or not the beast feeding is as effective as using a form of birth control that's more reliable.
I recommend that, after the first eight weeks of breast feeding, that you talk to your doctor or nurse practitioner about a supplemental form of birth control so that you don't have 2 babies in the course of a 12-month period. It can happen even if you are breast feeding. I don't know your body as well as you, and you don't know your body as well as you think you do. So after the first eight weeks of consistent breast feeding, without giving formula and without giving bottles, I would still use a birth control method that's consistent with effectiveness and compatible with breast feeding