I suspect that women whose menstrual periods return by 3-4 months postpartum are those who are nursing infrequently, using pacifiers, maybe using supplementary bottles. When control of milk production switches from mostly endocrine (prolactin based) to mostly autocrine (demand based) at 3-4 months postpartum, those moms who haven't been nursing a lot in the early months often experience a dramatic drop in supply. For most women (most, not all) the menstrual periods return when the baby's nursing frequency drops low enough to signal to the mother's body that the baby is old enough that it is OK to go ahead and get pregnant again for the next baby. Under baby-controlled breastfeeding and no pacifiers/fingers/bottles/cribs, this won't occur for several years for most women. Just like a woman whose baby has died at birth doesn't experience any lactational amenorrhea, neither does a woman who is completely bottle-feeding. And when women don't nurse very often in the first few months, their supply may *seem* adequate as long as the supply is determined by the prolactin, but their supply doesn't hold up when the system switches over to autocrine control based on milk removal. Katherine A. Dettwyler, Ph.D. Associate Professor of Anthropology and Nutrition Texas A&M University mailto:[log in to unmask]