I would never rely on breastfeeding as birth control at any time. If you have a big baby who begins sleeping long stretches at night early on, you lose that "protection" as early as 6 weeks. After my DD6, who was a champion nurser, exclusively breastfead for more than 6 months, I got my period at 3-mos. post-partum. Even though she was nursing, my mom got pregnant (and miscarried) 3 months post-partum. Same happened to my step-mom, except she actually had the baby, so they are just over 12 months apart. (Around here we call those Irish Twins.)

Even my experienced midwife assured me that exclusive, on-demand breastfeeding would prevent pregnancy. I told her I would not be taking that chance! After all, you have to ovulate to have a period, so you won't know when you've started until it is too late. Just don't do it!