This is on the Memorial Sloan-Kettering Herb site and mentions one animal study with possible increased progesterone production. See the summary below followed by the link.
Mechanism of Action
Cordyceps stimulates the number of T helper cells, prolongs the survival of lymphocytes, enhances TNF-alpha and interleukin 1 production, and increases the activity of natural killer cells in cultured rat Kupffer cells (
3). Enhanced proliferation of erythroid progenitor cell in the bone marrow of mice is also shown (
8). One study suggests that cordyceps can stimulate progesterone production in animal cells (
5). Another study shows that cordyceps may be effective against tumor celIs by down-regulating MHC class II antigen expression (
7). In addition, anecdotal data suggest reduction of cyclosporin and aminoglycoside-induced renal toxicity, although the mechanism of action is not known (
4).
http://www.mskcc.org/mskcc/html/69193.cfm