The real question is...Does anybody?
http://www.basskilleronline.com/finaplixfaqs.html#26
http://www.muscletalk.co.uk/oestrogen-progesterone.asp
http://www.steroidology.com/forum/showthread/t-828.html
http://www.steroidology.com/forum/showthread/t-21.html
Progestational Activity
It has been reported in other bodybuilding literature that trenbolone does not exhibit any activity as a progestin in the body. I am not certain where this belief originated, as trenbolone does appear to exhibit the classic progesterone receptor binding ability that is characteristic of nandrolone and its derivatives. One study looking at the bovine uterine progesterone receptor for example found trenbolone to be a very potent binder, startlingly even more so than progesterone itself (4). Another looking at the binding of various compounds to the androgen, estrogen, progestin, mineral corticoid and glucocorticoid receptors found trenbolone to be a more potent binder of the progestin receptor than nandrolone (5), a steroid normally noted for its usual activity in this regard. What does this mean for trenbolone? I dont think it really means that much. Trenbolone clearly doesnt cause gyno, water retention or fat buildup, which one might attribute to estrogenic or progestational activity. So whatever slight action it does have as a progestin on paper doesnt amount to all that much in the real world. The absence of estrogen may be a significant factor, as progesterone is believed to cause gyno by enhancing estrogens stimulation of mammary gland growth (6). Perhaps when trenbolone is taken with other aromatizable compounds it could affect a persons sensitivity level to gyno and water/fat retention. This seems logical, at least in a technical sense, although admittedly I have seen no evidence to support this.
above from muscle monthly 2001 tren part 1