[quote=TridentineFan]The average duration of breastfeeding amenorrhea in this study was 14.5 months. However, note that this is ecological breastfeeding (breast milk is only source of food for ~6 months, then is still a source of food as desired till child self-weans, mother breastfeeds throughout the night, all comfort is provided at the breast and not through pacifiers).
With “non-ecological” breastfeeding (for instance, if the baby sleeps in a crib, or uses a pacifier, or is sometimes bottle-fed expressed milk), the average duration of lactational amenorrhea seems to be about 10 months. This estimate comes from research done by the Kippleys (founders of CCL) and others, and it matches with the experience of women in my social circle.
This doesn’t necessary correlate with the return of fertility. As TridentineFan mentioned, a woman can ovulate before her first period. On the other hand, it’s also common for a nursing woman to remain infertile for some months after the return of her cycle, due to anovulatory cycles or a deficient luteal phase. (In the latter case, a baby may be conceived, but not be able to implant. Sad to think about, but evidently part of God’s plan.)
Of course, women’s bodies can behave very differently from the average. If a woman has a strong reason to space pregnancies, such as recovery from medical problems related to the birth, she’d be well-advised to practice a systematic form of NFP starting a few weeks postpartum. Methods that are based on external observation of the mucus sign (e.g. Creighton, Billings, and Marquette) are a simple and reliable way to predict the first ovulation, whether the woman is breastfeeding fully, partially, “ecologically,” or whatever.