I got pregnant while breastfeeding my first child and so did my sisiter, my periods had come back, so I knew there was a risk for me, my sisters had not returned before she got pregnant. 15 months between births. My periods returned at 6 weeks, following my second pregnancy despite exclusive breastfeeding, I am in the 2% that ovulation returns despite everything. So it is not a method of contraception that I recommend.
Also the PP is correct in saying that you can ovulate 2 weeks before your periods return (some women while breastfeeding have annovulatory periods but not all).
Lactational Amenorrhea Method (LAM)
The most common myth surrounding the use of contraception in lactating women is that lactation alone cannot be depended on to prevent pregnancy. The Lactational Amenorrhea Method (LAM) has been found to be better than 98 percent effective.(1) It has been used in a wide variety of settings, cultures, socioeconomic groups, and healthcare venues. This method is a good choice for the patient who prefers a natural method or does not want to take hormonal medication in the early months postpartum while she is exclusively nursing her baby.
Three main criteria must be met in order for LAM to be effective: 1) the baby should not be receiving any supplemental foods or artificial infant formula, 2) the baby must be less than 6 months old, and 3) the mother must not have resumed her menstrual cycle.
Ovulation in the non-lactating woman may occur as early as three weeks postpartum. The risk of ovulation in the high-frequency breastfeeding, amenorrheic woman is less than one to two percent.(2) Frequent nursing and/or pumping stimulates prolactin levels, which in turn suppress the surge of the follicle-stimulating and luteinizing hormones so that effective ovulation does not occur. Once the baby is six months old, it is more likely his diet will include foods other than human milk; he will breastfeed less and ovulation is more likely to occur.
LAM is not effective for the patient who plans to give supplemental feedings. Once the baby is over six months of age, the mother should plan on using an additional method of contraception if she wants to avoid pregnancy.
(from www.llli.org/ba/nov01.html)