Morgan - How about Kellymom http://www.kellymom.com/bf/supply/letdown.html.
"In extreme situations of stress or crisis, the release of extra adrenaline in the mother's system (the "fight or flight" response) can reduce or block the hormones which affect let-down."
You can find many citations under "Adrenaline and human let down" in Google. For most you need access to the journals.
Warmly, Toby Gish RN, IBCLC (Haifa, Israel)
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