[Cornish_Celt]

Delayed gyno is caused by an estrogen rebound. This is pretty common with compounds that don't convert to estrogen themselves. On cycle, prohormones and designer steroids can cause suppression of endogenous testosterone production, meaning low test on cycle. Less test levels means less aromatizing into estrogen, so estrogen levels are low as well. During post-cycle, as test levels return, estrogen can return in a "rebounding" spike that can cause gyno. This seems more common when an AI is used for PCT as this keeps estrogen levels suppressed even longer; when the AI is removed, estrogen levels spike up dramatically and cause gyno.