I have encountered contradictory attributions to the causal factor of midcycle FSH surge. Whereas some sources attribute it to the relatively high estrogen levels (1), others say it is because of increasing amounts of progesterone (2). I am confused.
(1) http://www2.hsc.wvu.edu/som/physio/classes/pcol260/classroom/notes/reproductive.htm
(2) https://www.ncbi.nlm.nih.gov/books/NBK279054/#!po=1.17188