So I am totally butting in here, not being an adoptive mom... but I thought I would just throw this out there bc nobody else has mentioned it.
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The composition of milk changes significantly over years of nursing. I would actually be kind of concerned about trying to supply all the nutritional needs of a newborn from breasts that think they are feeding a 5-year-old.
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I don't know of any studies of this in humans but there are a lot of them in dairy cows and goats, and it's pretty clear that the volume and nutritional content of the milk produced is optimized if there is a dry period during pregnancy, during which the tissue can start over in lactogenesis I.
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http://www.ncbi.nlm.nih.gov/pubmed/16476176
http://www.ncbi.nlm.nih.gov/pubmed/18420610
http://www.ncbi.nlm.nih.gov/pubmed/16162527
http://www.ncbi.nlm.nih.gov/pubmed/18024739
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I have also heard anecdotally that back when wet-nursing was common, a significant reason for higher mortality rates in wet-nursed infants was because they typically received the milk of a mother whose own baby was significantly older. And that milk banks will not accept donations from mothers whose last delivery was over a year ago.
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I really admire those of you who are willing to go through so much to provide the best for your adoptive babies, but I just wanted to throw it out there that some proportion of donor milk from a mom with an infant under 1 y/o might be preferable to full breastfeeding from a mom whose last pregnancy was over 5 years ago.
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Alternatively, if you do want to fully breastfeed your adoptive infant, it might be more optimal to wean your 5 y/o and take one of the hormonal protocols that allow your breast tissue to re-enter lactogenesis I, so that you generate more infant-appropriate milk (and most likely a better supply as well).
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Good luck!!