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
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